MUNICIPAL INVESTMENT TRUST FD INSURED SER 401 DEF ASSET FDS
485BPOS, 2001-01-17
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    AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON JANUARY 17, 2001

                                                      REGISTRATION NO. 333-65309
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------

                       SECURITIES AND EXCHANGE COMMISSION

                             WASHINGTON, D.C. 20549
                       ---------------------------------

                         POST-EFFECTIVE AMENDMENT NO. 2
                                       TO
                                    FORM S-6
                       ---------------------------------

                   FOR REGISTRATION UNDER THE SECURITIES ACT
                    OF 1933 OF SECURITIES OF UNIT INVESTMENT
                        TRUSTS REGISTERED ON FORM N-8B-2

                       ---------------------------------

A. EXACT NAME OF TRUST:
                        MUNICIPAL INVESTMENT TRUST FUND
                              INSURED SERIES--401
                              DEFINED ASSET FUNDS

B. NAME OF DEPOSITOR:

                   MERRILL LYNCH, PIERCE, FENNER & SMITH INC.
                           SALOMON SMITH BARNEY INC.
                            PAINEWEBBER INCORPORATED
                           DEAN WITTER REYNOLDS INC.

C. COMPLETE ADDRESSES OF DEPOSITORS' PRINCIPAL EXECUTIVE OFFICES:

<TABLE>
<S>                        <C>                        <C>
 MERRILL LYNCH, PIERCE,                               SALOMON SMITH BARNEY INC.
     FENNER & SMITH                                         388 GREENWICH
      INCORPORATED                                       STREET--23RD FLOOR
   DEFINED ASSET FUNDS                                   NEW YORK, NY 10013
  POST OFFICE BOX 9051
PRINCETON, NJ 08543-9051
</TABLE>

<TABLE>
<S>                        <C>                        <C>
PAINEWEBBER INCORPORATED                              DEAN WITTER REYNOLDS INC.
   1285 AVENUE OF THE                                      TWO WORLD TRADE
        AMERICAS                                         CENTER--59TH FLOOR
   NEW YORK, NY 10019                                    NEW YORK, NY 10048
</TABLE>

D. NAMES AND COMPLETE ADDRESSES OF AGENTS FOR SERVICE:

<TABLE>
<CAPTION>

<S>                        <C>                        <C>
  TERESA KONCICK, ESQ.                                    MICHAEL KOCHMANN
      P.O. BOX 9051                                     388 GREENWICH STREET
PRINCETON, NJ 08543-9051                                 NEW YORK, NY 10013
                                  COPIES TO:
    ROBERT E. HOLLEY         NORA M. JORDAN, ESQ.        DOUGLAS LOWE, ESQ.
    1200 HARBOR BLVD.        450 LEXINGTON AVENUE     DEAN WITTER REYNOLDS INC.
   WEEHAWKEN, NJ 07087        NEW YORK, NY 10017           TWO WORLD TRADE
                                                         CENTER--59TH FLOOR
                                                         NEW YORK, NY 10048
</TABLE>

The issuer has registered an indefinite number of Units under the Securities Act
of 1933 pursuant to Rule 24f-2 and will file the Rule 24f-2 Notice for the most
recent fiscal year in March, 2001.

Check box if it is proposed that this filing will become effective on January
26, 2001 pursuant to paragraph (b) of Rule 485.  /X/
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
<PAGE>

                           DEFINED ASSET FUNDS-REGISTERED TRADEMARK-
                           ----------------------------------------------------

                           MUNICIPAL INVESTMENT TRUST FUND
                           INSURED SERIES--401
                           (A UNIT INVESTMENT TRUST)

                           -  PORTFOLIO OF INSURED LONG TERM MUNICIPAL BONDS
                           -  DESIGNED TO BE FREE OF REGULAR FEDERAL INCOME TAX
                           -  INCOME DISTRIBUTIONS TWICE A YEAR

SPONSORS:
MERRILL LYNCH,             -----------------------------------------------------
PIERCE, FENNER & SMITH     The Securities and Exchange Commission has not
INCORPORATED               approved or disapproved these Securities or passed
SALOMON SMITH BARNEY INC.  upon the adequacy of this prospectus. Any
PAINEWEBBER INCORPORATED   representation to the contrary is a criminal offense.
DEAN WITTER REYNOLDS INC.  Prospectus dated January 26, 2001.

<PAGE>
--------------------------------------------------------------------------------

Defined Asset Funds-Registered Trademark-
Defined Asset Funds-Registered Trademark- is America's oldest and largest family
of unit investment trusts, with over $160 billion sponsored over the last 28
years. Defined Asset Funds has been a leader in unit investment trust research
and product innovation. Our family of Funds helps investors work toward their
financial goals with a full range of quality investments, including municipal,
corporate and government bond portfolios, as well as domestic and international
equity portfolios.

Defined Asset Funds offer a number of advantages:
   - A disciplined strategy of buying and holding with a long-term view is the
     cornerstone of Defined Asset Funds.
   - Fixed portfolio: Defined Funds follow a buy and hold investment strategy;
     funds are not managed and portfolio changes are limited.
   - Defined Portfolios: We choose the stocks and bonds in advance, so you know
     what you're investing in.
   - Professional research: Our dedicated research team seeks out stocks or
     bonds appropriate for a particular fund's objectives.
   - Ongoing supervision: We monitor each portfolio on an ongoing basis.

No matter what your investment goals, tolerance for risk or time horizon,
there's probably a Defined Asset Fund that suits your investment style. Your
financial professional can help you select a Defined Asset Fund that works best
for your investment portfolio.

THE FINANCIAL INFORMATION IN THIS PROSPECTUS IS AS OF THE EVALUATION DATE,
OCTOBER 31, 2000.

CONTENTS

<TABLE>
<CAPTION>
                                                    PAGE
                                                    ----
<S>                                                 <C>
Risk/Return Summary...............................    3
What You Can Expect From Your Investment..........    7
  Income Twice a Year.............................    7
  Return Figures..................................    7
  Records and Reports.............................    7
The Risks You Face................................    8
  Interest Rate Risk..............................    8
  Call Risk.......................................    8
  Reduced Diversification Risk....................    8
  Liquidity Risk..................................    8
  Concentration Risk..............................    8
  Bond Quality Risk...............................    9
  Insurance Related Risk..........................    9
  Litigation and Legislation Risks................    9
Selling or Exchanging Units.......................    9
  Sponsors' Secondary Market......................    9
  Selling Units to the Trustee....................   10
  Exchange Option.................................   10
How The Fund Works................................   11
  Pricing.........................................   11
  Evaluations.....................................   11
  Income..........................................   11
  Expenses........................................   11
  Portfolio Changes...............................   12
  Fund Termination................................   12
  Certificates....................................   13
  Trust Indenture.................................   13
  Legal Opinion...................................   14
  Auditors........................................   14
  Sponsors........................................   14
  Trustee.........................................   14
  Underwriters' and Sponsors' Profits.............   14
  Public Distribution.............................   15
  Code of Ethics..................................   15
Taxes.............................................   15
Supplemental Information..........................   16
Financial Statements..............................  D-1
</TABLE>

                                       2
<PAGE>
--------------------------------------------------------------------------------

RISK/RETURN SUMMARY

 1. WHAT IS THE FUND'S OBJECTIVE?
   The Fund seeks interest income that is exempt from regular federal income
   taxes by investing in a fixed portfolio consisting primarily of insured
   municipal revenue bonds.

 2. WHAT ARE MUNICIPAL REVENUE BONDS?
   Municipal revenue bonds are bonds issued by states, municipalities and public
   authorities to finance the cost of buying, building or improving various
   projects intended to generate revenue, such as airports, health care
   facilities, housing and municipal electric, water and sewer utilities.
   Generally, payments on these bonds depend solely on the revenues generated by
   the projects, excise taxes or state appropriations, and are not backed by the
   government's taxing power.

 3. WHAT IS THE FUND'S INVESTMENT STRATEGY?
 - The Fund plans to hold to maturity 14 insured long-term tax-exempt municipal
   bonds, including some short-term bonds reserved to pay the deferred sales
   charge, with a current aggregate face amount of $8,375,000.
 - The Fund is a unit investment trust which means that, unlike a mutual fund,
   the Fund's portfolio is not managed.
 - The bonds are rated AAA or Aaa by Standard & Poor's, Moody's or Fitch.
 - Many of the bonds can be called at a premium declining over time to par
   value. Some bonds may be called earlier at par for extraordinary reasons.
 - 100% of the bonds are insured by insurance companies. Insurance guarantees
   timely payments of principal and interest on the bonds (but not Fund units or
   the market value of the bonds before they mature).

   The Portfolio consists of municipal bonds of the following types:

<TABLE>
<CAPTION>
                                                    APPROXIMATE
                                                     PORTFOLIO
                                                    PERCENTAGE
<S>                                                 <C>
-Convention Centers                                       12%
-General Obligation                                        2%
-Hospitals/Health Care                                    41%
-Housing                                                  12%
-Lease Rental                                              4%
-Special Tax                                               9%
-Municipal Electric Utilities                             20%
</TABLE>

 4. WHAT ARE THE SIGNIFICANT RISKS?
   YOU CAN LOSE MONEY BY INVESTING IN THE FUND. THIS CAN HAPPEN FOR VARIOUS
   REASONS, INCLUDING:
 - Rising interest rates, an issuer's worsening financial condition or a drop in
   bond ratings can reduce the price of your units.
 - Because the Fund is concentrated in hospital/health care bonds, adverse
   developments in this sector may affect the value of your units.
 - Assuming no changes in interest rates, when you sell your units, they will
   generally be worth less than your cost because your cost included a sales
   fee.
 - The Fund will receive early returns of principal if bonds are called or sold
   before they mature. If this happens your income will decline and you may not
   be able to reinvest the money you receive at as high a yield or as long a
   maturity.

 5. IS THIS FUND APPROPRIATE FOR YOU?
   Yes, if you want income free from regular federal income tax. You will
   benefit from a professionally selected and supervised portfolio whose risk is
   reduced by investing in bonds of several different issuers.
   The Fund is NOT appropriate for you if you want a speculative investment that
   changes to take advantage of market movements, if you do not want a
   tax-advantaged investment or if you cannot tolerate any risk.

                                       3
<PAGE>
                              DEFINING YOUR INCOME

<TABLE>
<S>                                                 <C>
WHAT YOU MAY EXPECT (Payable on the 25th day of
each June and December):
Regular semi-annual Income per 1,000 units          $24.40
Annual Income per 1,000 units                       $48.81
RECORD DAY: 10th day of each June and December
THESE FIGURES ARE ESTIMATES ON THE EVALUATION DATE; ACTUAL
PAYMENTS MAY VARY.
</TABLE>

 6. WHAT ARE THE FUND'S FEES AND EXPENSES?
   This table shows the costs and expenses you may pay, directly or indirectly,
   when you invest in the Fund.

<TABLE>
<S>                                                 <C>
INVESTOR FEES
Maximum Sales Fee (Load) on new
purchases (as a percentage of
$1,000 invested)                                    2.90%
</TABLE>

   You will pay an up-front sales fee of    % as well as a deferred sales fee of
   $3.75 per unit quarterly November, February, May and August, through
            200 .
   Employees of some of the Sponsors and their affiliates may be charged a
   reduced sales fee of no less than $5.00 per unit.

   The maximum sales fee is reduced if you invest at least $100,000, as follows:

<TABLE>
<CAPTION>
                                                    YOUR MAXIMUM
                                                     SALES FEE
                  IF YOU INVEST:                      WILL BE:
                  --------------                    ------------
<S>                                                 <C>
Less than $100,000                                        2.90%
$100,000 to $249,999                                      2.65%
$250,000 to $499,999                                      2.40%
$500,000 to $999,999                                      2.15%
$1,000,000 and over                                       1.90%

Maximum Exchange Fee                                      1.90%
</TABLE>

   ESTIMATED ANNUAL FUND OPERATING EXPENSES

<TABLE>
<CAPTION>
                                                        AMOUNT
                                                    PER 1,000 UNITS
                                                    ---------------
<S>                                                 <C>
Trustee's Fee                                            $0.63
Portfolio Supervision,
 Bookkeeping and
 Administrative Fees
 (including updating
 expenses)                                               $0.50
Evaluator's Fee                                          $0.13
Other Operating Expenses                                 $0.46
                                                         -----
TOTAL                                                    $1.72
</TABLE>

   The Sponsors historically paid updating expenses.

 7. HOW HAVE SIMILAR FUNDS PERFORMED IN THE PAST?
   IN THE FOLLOWING CHART WE SHOW PAST PERFORMANCE OF PRIOR MUNICIPAL INSURED
   SERIES, WHICH HAD THE SAME INVESTMENT OBJECTIVES, STRATEGIES AND TYPES OF
   BONDS AS THIS FUND. These prior Insured Series were offered after 1987 and
   were outstanding on September 30, 2000. OF COURSE, PAST PERFORMANCE OF PRIOR
   SERIES IS NO GUARANTEE OF FUTURE RESULTS OF THIS FUND.

   AVERAGE ANNUAL COMPOUND TOTAL RETURNS
                    FOR PRIOR SERIES
     REFLECTING ALL EXPENSES. FOR PERIODS ENDED 9/30/00.

<TABLE>
<CAPTION>
                       WITH SALES FEE                          NO SALES FEE
               1 YEAR      5 YEARS     10 YEARS       1 YEAR      5 YEARS     10 YEARS
<S>         <C>           <C>         <C>          <C>           <C>         <C>
----------------------------------------------------------------------------------------
High               7.33%      5.34%       6.51%           7.84%      6.51%       7.04%
Average            3.84       4.40        6.20            6.18       5.49        6.78
Low                2.36       2.98        5.80            4.03       3.80        6.39
----------------------------------------------------------------------------------------
Average
Sales fee          2.08%      5.28%       5.69%
----------------------------------------------------------------------------------------
</TABLE>

NOTE: ALL RETURNS REPRESENT CHANGES IN UNIT PRICE WITH DISTRIBUTIONS REINVESTED
INTO THE MUNICIPAL FUND INVESTMENT ACCUMULATION PROGRAM.

 8. IS THE FUND MANAGED?
   Unlike a mutual fund, the Fund is not managed and bonds are not sold because
   of market changes. Rather, experienced Defined Asset Funds financial analysts
   regularly review the bonds in the Fund. The Fund may sell a bond if certain
   adverse credit or other conditions exist.

 9. HOW DO I BUY UNITS?
   The minimum investment is one unit.

   You can buy units from any of the Sponsors and other broker-dealers. The
   Sponsors are listed later in this prospectus. Some banks may offer units for
   sale through special arrangements with the Sponsors, although certain legal
   restrictions may apply.

                                       4
<PAGE>

<TABLE>
<S>                                                 <C>
UNIT PRICE PER 1,000 UNITS                          $934.59
(as of October 31, 2000)
</TABLE>

   Unit price is based on the net asset value of the Fund plus the sales fee. An
   amount equal to any principal cash, as well as net accrued but undistributed
   interest on the unit, is added to the unit price. An independent evaluator
   prices the bonds at 3:30 p.m. Eastern time every business day. Unit price
   changes every day with changes in the prices of the bonds in the Fund.

10. HOW DO I SELL UNITS?
   You may sell your units at any time to any Sponsor or the Trustee for the net
   asset value determined at the close of business on the date of sale, less any
   remaining deferred sales fee. You will not pay any other fee when you sell
   your units.

11. HOW ARE DISTRIBUTIONS MADE AND TAXED?
   The Fund pays income twice a year. In the opinion of bond counsel when each
   bond was issued, interest on the bonds in this Fund is generally 100% exempt
   from regular federal income tax.

   A portion of the income may also be exempt from state and local personal
   income taxes, depending on where you live.

   You will also receive principal payments if bonds are sold or called or
   mature, when the cash available is more than $10.00 per 1,000 units. You will
   be subject to tax on any gain realized by the Fund on the disposition of
   bonds.

12. WHAT OTHER SERVICES ARE AVAILABLE?

   REINVESTMENT
   You will receive your income in cash unless you choose to compound your
   income by reinvesting with no sales fee in the Municipal Fund Investment
   Accumulation Program, Inc. This program is an open-end mutual fund with a
   comparable investment objective, but those bonds generally will not be
   insured. Income from this program will generally be subject to state and
   local income taxes. FOR MORE COMPLETE INFORMATION ABOUT THE PROGRAM,
   INCLUDING CHARGES AND FEES, ASK THE TRUSTEE FOR THE PROGRAM'S PROSPECTUS.
   READ IT CAREFULLY BEFORE YOU INVEST. THE TRUSTEE MUST RECEIVE YOUR WRITTEN
   ELECTION TO REINVEST AT LEAST 10 DAYS BEFORE THE RECORD DAY OF AN INCOME
   PAYMENT.

   EXCHANGE PRIVILEGES
   You may exchange units of this Fund for units of certain other Defined Asset
   Funds. You may also exchange into this Fund from certain other funds. We
   charge a reduced sales fee on exchanges.

                                       5
<PAGE>
--------------------------------------------------------------------------------
    TAX-FREE VS. TAXABLE INCOME: A COMPARISON OF TAXABLE AND TAX-FREE YIELDS

<TABLE>
<CAPTION>
                                        EFFECTIVE
           TAXABLE INCOME 2001*           % TAX                            TAX-FREE YIELD OF
    SINGLE RETURN       JOINT RETURN     BRACKET     3%     3.5%     4%     4.5%     5%      5.5%      6%      6.5%
                                                                  IS EQUIVALENT TO A TAXABLE YIELD OF
    -----------------------------------------------------------------------------------------------------------------
    <S>               <C>               <C>        <C>     <C>     <C>     <C>     <C>      <C>      <C>      <C>
    $      0- 26,250  $      0- 43,850     15.00    3.53    4.12    4.71    5.29     5.88     6.47     7.06     7.65
    -----------------------------------------------------------------------------------------------------------------
    $ 26,251- 63,550  $ 43,851-105,950     28.00    4.17    4.86    5.56    6.25     6.94     7.64     8.33     9.03
    -----------------------------------------------------------------------------------------------------------------
    $ 63,551-132,600  $105,951-161,450     31.00    4.35    5.07    5.80    6.52     7.25     7.97     8.70     9.42
    -----------------------------------------------------------------------------------------------------------------
    $132,601-288,350  $161,451-288,350     36.00    4.69    5.47    6.25    7.03     7.81     8.59     9.38    10.16
    -----------------------------------------------------------------------------------------------------------------
       OVER $288,350     OVER $288,350     39.60    4.97    5.79    6.62    7.45     8.28     9.11     9.93    10.76
    -----------------------------------------------------------------------------------------------------------------
</TABLE>

To compare the yield of a taxable security with the yield of a federally
tax-free security, find your taxable income and read across. The table
incorporates 2001 federal income tax rates and assumes that all income would
otherwise be taxed at a U.S. investor's highest tax rate. Yield figures are for
example only.

* Based upon net amount subject to federal income tax after deductions and
exemptions. This table does not reflect the possible effect of other tax
factors, such as alternative minimum tax, personal exemptions, the phase out of
exemptions, itemized deductions, the possible partial disallowance of deductions
or state or local taxation. Consequently, you should consult your own tax
advisers in this regard.

                                       6
<PAGE>
WHAT YOU CAN EXPECT FROM YOUR INVESTMENT

INCOME TWICE A YEAR

The Fund will pay you regular semi-annual income. Your income may vary because
of:

  - elimination of one or more bonds from the Fund's portfolio because of calls,
    redemptions or sales;
  - a change in the Fund's expenses; or
  - the failure by a bond's issuer to pay interest.

Changes in interest rates generally will not affect your income because the
portfolio is fixed.

Along with your income, you will receive your share of any available bond
principal.

RETURN FIGURES

We cannot predict your actual return, which will vary with unit price, how long
you hold your investment and changes in the portfolio, interest income and
expenses.

ESTIMATED CURRENT RETURN equals the estimated annual cash to be received from
the bonds in the Fund less estimated annual Fund expenses, divided by the Unit
Price (including the maximum sales fee):

<TABLE>
<S>               <C><C>
Estimated Annual  -     Estimated
Interest Income      Annual Expenses
------------------------------------
             Unit Price
</TABLE>

ESTIMATED LONG TERM RETURN is a measure of the estimated return over the
estimated life of the Fund. Unlike Estimated Current Return, Estimated Long Term
Return reflects maturities, discounts and premiums of the bonds in the Fund. It
is an average of the yields to maturity (or in certain cases, to an earlier call
date) of the individual bonds in the portfolio, adjusted to reflect the Fund's
maximum sales fee and estimated expenses. We calculate the average yield for the
portfolio by weighting each bond's yield by its market value and the time
remaining to the call or maturity date.

Yields on individual bonds depend on many factors including general conditions
of the bond markets, the size of a particular offering and the maturity and
quality rating of the particular issues. Yields can vary among bonds with
similar maturities, coupons and ratings.

These return quotations are designed to be comparative rather than predictive.

RECORDS AND REPORTS

You will receive:

- a monthly statement of income payments and any principal payments;
- a notice from the Trustee when new bonds are deposited in exchange or
  substitution for bonds originally deposited;
- an annual report on Fund activity; and
- annual tax information. THIS WILL ALSO BE SENT TO THE IRS. YOU MUST REPORT THE
  AMOUNT OF TAX-EXEMPT INTEREST RECEIVED DURING THE YEAR.

You may request:

- copies of bond evaluations to enable you to comply with federal and state tax
  reporting requirements; and
- audited financial statements of the Fund.

You may inspect records of Fund transactions at the Trustee's office during
regular business hours.

                                       7
<PAGE>
THE RISKS YOU FACE

INTEREST RATE RISK

Investing involves risks, including the risk that your investment will decline
in value if interest rates rise. Generally, bonds with longer maturities will
change in value more than bonds with shorter maturities. Bonds in the Fund are
more likely to be called when interest rates decline. This would result in early
returns of principal to you and may result in early termination of the Fund. Of
course, we cannot predict how interest rates may change.

CALL RISK

Many bonds can be prepaid or "called" by the issuer before their stated
maturity.

For example, an issuer might call its bonds if it no longer needs the money for
the original purpose or, during periods of falling interest rates, if the
issuer's bonds have a coupon higher than current market rates. If the bonds are
called, your income will decline and you may not be able to reinvest the money
you receive at as high a yield or as long a maturity. An early call at par of a
premium bond will reduce your return.

REDUCED DIVERSIFICATION RISK

If many investors sell their units, the Fund will have to sell bonds. This could
reduce the diversification of your investment and increase your share of Fund
expenses.

LIQUIDITY RISK

The bonds will generally trade in the over-the-counter market. We cannot assure
you that a liquid trading market will exist, especially since current law may
restrict the Fund from selling bonds to any Sponsor. The value of the bonds, and
of your investment, may be reduced if trading in bonds is limited or absent.

CONCENTRATION RISK

When a certain type of bond makes up 25% or more of the portfolio, the Fund is
said to be "concentrated" in that bond type, which makes the Fund less
diversified.

Here is what you should know about the Fund's concentration in hospital and
health care bonds:

  - payment for these bonds depends on revenues from private third-party payors
    and government programs, including Medicare and Medicaid, which have
    generally undertaken cost containment measures to limit payments to
    healthcare providers;
  - hospitals face increasing competition resulting from hospital mergers and
    affiliations;
  - hospitals need to reduce costs as HMOs increase market penetration and
    hospital supply and drug companies raise prices;
  - hospitals and health care providers are subject to various legal claims by
    patients and others and are adversely affected by increasing costs of
    insurance;
  - many hospitals are aggressively buying physician practices and assuming risk
    contracts to gain market share. If revenues do not increase accordingly,
    this practice could reduce profits;
  - Medicare is changing its reimbursement system for nursing homes. Many
    nursing home providers are not sure how they will be treated. In many cases,
    the

                                       8
<PAGE>
    providers may receive lower reimbursements and these would have to cut
    expenses to maintain profitability; and
  - most retirement/nursing home providers rely on entrance fees for operating
    revenues. If people live longer than expected and turnover is lower than
    budgeted, operating revenues would be adversely affected by less than
    expected entrance fees.

Changes to the portfolio from bond redemptions, maturities and sales may affect
the Fund's concentration over time.

BOND QUALITY RISK

A reduction in a bond's rating may decrease its value and, indirectly, the value
of your investment in the Fund.

INSURANCE RELATED RISK

The bonds are backed by insurance companies (as shown under Defined Portfolio).
Insurance policies generally make payments only according to a bond's original
payment schedule and do not make early payments when a bond defaults or becomes
taxable. Although the federal government does not regulate the insurance
business, various state laws and federal initiatives and tax law changes could
significantly affect the insurance business. The claims-paying ability of the
insurance companies is generally rated A or better by Standard & Poor's or
another nationally recognized rating organization. The insurance company ratings
are subject to change at any time at the discretion of the rating agencies.

LITIGATION AND LEGISLATION RISKS

We do not know of any pending litigation that might have a material adverse
effect upon the Fund.
Future tax legislation could affect the value of the portfolio by:

  - limiting real property taxes,
  - reducing tax rates,
  - imposing a flat or other form of tax, or
  - exempting investment income from tax.

SELLING OR EXCHANGING UNITS

You can sell your units at any time for a price based on net asset value. Your
net asset value is calculated each business day by:

  - ADDING the value of the bonds, net accrued interest, cash and any other Fund
    assets;
  - SUBTRACTING accrued but unpaid Fund expenses, unreimbursed Trustee advances,
    cash held to buy back units or for distribution to investors and any other
    Fund liabilities; and
  - DIVIDING the result by the number of outstanding units.

Your net asset value when you sell may be more or less than your cost because of
sales fees, market movements and changes in the portfolio.

If you sell your units before the final deferred sales fee installment, the
amount of any remaining installments will be deducted from your proceeds.

SPONSORS' SECONDARY MARKET

While we are not obligated to do so, we will buy back units at net asset value
without any other fee or charge other than any

                                       9
<PAGE>
remaining deferred sales fee. We may resell the units to other buyers or to the
Trustee. You should consult your financial professional for current market
prices to determine if other broker-dealers or banks are offering higher prices.

We have maintained a secondary market continuously for over 28 years, but we
could discontinue it without prior notice for any business reason.

SELLING UNITS TO THE TRUSTEE

Regardless of whether we maintain a secondary market, you can sell your units to
the Trustee at any time by sending the Trustee a letter (with any outstanding
certificates if you hold unit certificates). You must properly endorse your
certificates (or execute a written transfer instrument with signatures
guaranteed by an eligible institution). Sometimes, additional documents are
needed such as a trust document, certificate of corporate authority, certificate
of death or appointment as executor, administrator or guardian.

Within seven days after your request and the necessary documents are received,
the Trustee will mail a check to you. Contact the Trustee for additional
information.

As long as we are maintaining a secondary market, the Trustee will sell your
units to us at a price based on net asset value. If there is no secondary
market, the Trustee may sell your units in the over-the-counter market for a
higher price, but it is not obligated to do so. In that case, you will receive
the net proceeds of the sale.

If the Fund does not have cash available to pay you for units you are selling,
the agent for the Sponsors will select bonds to be sold. Bonds will be selected
based on market and credit factors. These sales could be made at times when the
bonds would not otherwise be sold and may result in your receiving less than the
unit par value and also reduce the size and diversity of the Fund.

If you acquire 25% or more of the outstanding units of the Fund and you sell
units with a value exceeding $250,000, the Trustee may choose to pay you "in
kind" by distributing bonds and cash with a total value equal to the price of
those units. The Trustee will try to distribute bonds in the portfolio pro rata,
but it reserves the right to distribute only one or a few bonds. The Trustee
will act as your agent in an in kind distribution and will either hold the bonds
for your account or sell them as you instruct. You must pay any transaction
costs as well as transfer and ongoing custodial fees on sales of bonds
distributed in kind.

There could be a delay in paying you for your units:

  - if the New York Stock Exchange is closed (other than customary weekend and
    holiday closings);
  - if the SEC determines that trading on the New York Stock Exchange is
    restricted or that an emergency exists making sale or evaluation of the
    bonds not reasonably practicable; and
  - for any other period permitted by SEC order.

EXCHANGE OPTION

You may exchange units of certain Defined Asset Funds for units of this Fund at
a maximum exchange fee of 1.90%. You may exchange units of this Fund for units
of

                                       10
<PAGE>
certain other funds at a reduced sales fee if your investment goals change. In
addition, you may exchange into this Fund from certain other Defined Asset Funds
and unit trusts. To exchange units, you should talk to your financial
professional about what funds are exchangeable, suitable and currently
available.

Normally, an exchange is taxable and you must recognize any gain or loss on the
exchange. However, the IRS may try to disallow a loss if the portfolios of the
two funds are not materially different; you should consult your own tax adviser.

We may amend or terminate this exchange option at any time without notice.

HOW THE FUND WORKS

PRICING

The price of a unit includes interest accrued on the bonds, less expenses, from
the most recent Record Day up to, but not including, the settlement date, which
is usually three business days after the purchase date of the unit.

A portion of the price of a unit consists of cash so that the Trustee can
provide you with regular income. When you sell your units you will receive your
share of this cash.

EVALUATIONS

An independent Evaluator values the bonds on each business day (excluding
Saturdays, Sundays and the following holidays as observed by the New York Stock
Exchange: New Year's Day, Presidents' Day, Martin Luther King, Jr. Day, Good
Friday, Memorial Day, Independence Day, Labor Day, Thanksgiving and Christmas).
Bond values are based on current bid or offer prices for the bonds or comparable
bonds. In the past, the difference between bid and offer prices of publicly
offered tax-exempt bonds has ranged from 0.5% of face amount on actively traded
issues to 3.5% on inactively traded issues; the difference has averaged between
1 and 2%.

INCOME

The Trustee credits interest to an Income Account and other receipts to a
Capital Account. The Trustee may establish a Reserve Account by withdrawing from
these accounts amounts it considers appropriate to pay any material liability.
These accounts do not bear interest.

EXPENSES

The Trustee is paid monthly. It also benefits when it holds cash for the Fund in
non-interest bearing accounts. The Trustee may also receive additional amounts:

  - to reimburse the Trustee for the Fund's operating expenses;
  - for extraordinary services and costs of indemnifying the Trustee and the
    Sponsors;
  - costs of actions taken to protect the Fund and other legal fees and
    expenses;
  - expenses for keeping the Fund's registration statement current; and
  - Fund termination expenses and any governmental charges.

The Sponsors are currently reimbursed up to 55 CENTS per $1,000 face amount
annually for providing portfolio supervisory, bookkeeping and administrative
services and for any other expenses properly chargeable to the

                                       11
<PAGE>
Fund. Legal, typesetting, electronic filing and regulatory filing fees and
expenses associated with updating the Portfolio's registration statement yearly
are also now chargeable to the Portfolio. While this fee may exceed the amount
of these costs and expenses attributable to this Fund, the total of these fees
for all Series of Defined Asset Funds will not exceed the aggregate amount
attributable to all of these Series for any calendar year. The Fund also pays
the Evaluator's fees.

The Trustee's, Sponsors' and Evaluator's fees may be adjusted for inflation
without investors' approval.

Quarterly deferred sales charges you owe are paid with interest and principal
from certain bonds. If these amounts are not enough, the rest will be paid out
of distributions to you from the Fund's Capital and Income Accounts.

The Sponsors will pay advertising and selling expenses at no charge to the Fund.
If Fund expenses exceed initial estimates, the Fund will owe the excess. The
Trustee has a lien on Fund assets to secure reimbursement of Fund expenses and
may sell bonds if cash is not available.

PORTFOLIO CHANGES

The Sponsors and Trustee are not liable for any default or defect in a bond.

Unlike a mutual fund, the portfolio is designed to remain intact and we may keep
bonds in the portfolio even if their credit quality declines or other adverse
financial circumstances occur. However, we may sell a bond in certain cases if
we believe that certain adverse credit conditions exist or if a bond becomes
taxable.

If we maintain a secondary market in units but are unable to sell the units that
we buy in the secondary market, we will redeem units, which may affect the
composition of the portfolio. Units offered in the secondary market may not
represent the same face amount of bonds that they did originally.

We decide whether or not to offer units for sale that we acquire in the
secondary market after reviewing:

  - diversity of the portfolio;
  - size of the Fund relative to its original size;
  - ratio of Fund expenses to income;
  - current and long-term returns;
  - degree to which units may be selling at a premium over par; and
  - cost of maintaining a current prospectus.

FUND TERMINATION

The Fund will terminate following the stated maturity or sale of the last bond
in the portfolio. The Fund may also terminate earlier with the consent of
investors holding 51% of the units or if total assets of the Fund have fallen
below 40% of the face amount of bonds deposited. We will decide whether to
terminate the Fund early based on the same factors used in deciding whether or
not to offer units in the secondary market.

When the Fund is about to terminate you will receive a notice, and you will be
unable to sell your units after that time. On or shortly before termination, we
will sell any remaining bonds, and you will receive your final distribution. Any
bond that cannot be

                                       12
<PAGE>
sold at a reasonable price may continue to be held by the Trustee in a
liquidating trust pending its final sale.

You will pay your share of the expenses associated with termination, including
brokerage costs in selling bonds. This may reduce the amount you receive as your
final distribution.

CERTIFICATES

Certificates for units are issued on request. You may transfer certificates by
complying with the requirements for redeeming certificates, described above. You
can replace lost or mutilated certificates by delivering satisfactory indemnity
and paying the associated costs.

TRUST INDENTURE

The Fund is a "unit investment trust" governed by a Trust Indenture, a contract
among the Sponsors, the Trustee and the Evaluator, which sets forth their duties
and obligations and your rights. A copy of the Indenture is available to you on
request to the Trustee. The following summarizes certain provisions of the
Indenture.

The Sponsors and the Trustee may amend the Indenture without your consent:

  - to cure ambiguities;
  - to correct or supplement any defective or inconsistent provision;
  - to make any amendment required by any governmental agency; or
  - to make other changes determined not to be materially adverse to your best
    interest (as determined by the Sponsors).

Investors holding 51% of the units may amend the Indenture. Every investor must
consent to any amendment that changes the 51% requirement. No amendment may
reduce your interest in the Fund without your written consent.

The Trustee may resign by notifying the Sponsors. The Sponsors may remove the
Trustee without your consent if:

  - it fails to perform its duties and the Sponsors determine that its
    replacement is in your best interest; or
  - it becomes incapable of acting or bankrupt or its affairs are taken over by
    public authorities.

Investors holding 51% of the units may remove the Trustee. The Evaluator may
resign or be removed by the Sponsors and the Trustee without the consent of
investors. The resignation or removal of either becomes effective when a
successor accepts appointment. The Sponsors will try to appoint a successor
promptly; however, if no successor has accepted within 30 days after notice of
resignation, the resigning Trustee or Evaluator may petition a court to appoint
a successor.

Any Sponsor may resign as long as one Sponsor with a net worth of $2 million
remains and agrees to the resignation. The remaining Sponsors and the Trustee
may appoint a replacement. If there is only one Sponsor and it fails to perform
its duties or becomes bankrupt the Trustee may:

  - remove it and appoint a replacement Sponsor;
  - liquidate the Fund; or
  - continue to act as Trustee without a Sponsor.

Merrill Lynch, Pierce, Fenner & Smith Incorporated acts as agent for the
Sponsors.

                                       13
<PAGE>
The Trust Indenture contains customary provisions limiting the liability of the
Trustee, the Sponsors and the Evaluator.

LEGAL OPINION

Davis Polk & Wardwell, 450 Lexington Avenue, New York, New York 10017, as
special counsel for the Sponsors, has given an opinion that the units are
validly issued.

AUDITORS

Deloitte & Touche LLP, 2 World Financial Center, New York, New York 10281,
independent accountants, audited the Statement of Condition included in this
prospectus.

SPONSORS

The Sponsors are:

MERRILL LYNCH, PIERCE, FENNER & SMITH INCORPORATED (a wholly-owned subsidiary of
Merrill Lynch & Co., Inc.)
P.O. Box 9051,
Princeton, NJ 08543-9051

SALOMON SMITH BARNEY INC. (an indirectly wholly-owned subsidiary of Citigroup
Inc.)
388 Greenwich Street--23rd Floor,
New York, NY 10013

DEAN WITTER REYNOLDS INC. (a principal operating subsidiary of Morgan Stanley
Dean Witter & Co.)
Two World Trade Center--59th Floor,
New York, NY 10048

PAINEWEBBER INCORPORATED (a wholly-owned subsidiary of PaineWebber Group Inc.)
1285 Avenue of the Americas,
New York, NY 10019

Each Sponsor is a Delaware corporation and it, or its predecessor, has acted as
sponsor to many unit investment trusts. As a registered broker-dealer each
Sponsor buys and sells securities (including investment company shares) for
others (including investment companies) and participates as an underwriter in
various selling groups.

TRUSTEE

The Chase Manhattan Bank, Unit Investment Trust Department, 4 New York
Plaza--6th Floor, New York, New York 10004, is the Trustee. It is supervised by
the Federal Deposit Insurance Corporation, the Board of Governors of the Federal
Reserve System and New York State banking authorities.

UNDERWRITERS' AND SPONSORS' PROFITS

Underwriters receive sales charges when they sell units. The Sponsors also
realized a profit or loss on the initial deposit of the bonds. Any cash made
available by you to the Sponsors before the settlement date for those units may
be used in the Sponsors' businesses to the extent permitted by federal law and
may benefit the Sponsors.

A Sponsor or Underwriter may realize profits or sustain losses on bonds in the
Fund which were acquired from underwriting syndicates of which it was a member.

In maintaining a secondary market, the Sponsors will also realize profits or
sustain losses in the amount of any difference between the prices at which they
buy units and the prices at which they resell or redeem them.

                                       14
<PAGE>
PUBLIC DISTRIBUTION

The Sponsors do not intend to qualify units for sale in any foreign countries.
This prospectus does not constitute an offer to sell units in any country where
units cannot lawfully be sold.

CODE OF ETHICS

The Fund and the Agent for the Sponsors have each adopted a code of ethics
requiring reporting of personal securities transactions by its employees with
access to information on Fund transactions. Subject to certain conditions, the
codes permit employees to invest in Fund securities for their own accounts. The
codes are designed to prevent fraud, deception and misconduct against the Fund
and to provide reasonable standards of conduct. These codes are on file with the
Commission and you may obtain a copy by contacting the Commission at the address
listed on the back cover of this prospectus.

TAXES

The following summary describes some of the important income tax consequences of
holding units. It assumes that you are not a dealer, financial institution,
insurance company or other investor with special circumstances or subject to
special rules. You should consult your own tax adviser about your particular
circumstances.

At the date of issue of each bond, counsel for the issuer delivered an opinion
to the effect that interest on the bond is exempt from regular federal income
tax. However, interest may be subject to state and local taxes and may be taken
into account in determining your preference items for alternative minumum tax
purposes. Neither we nor our counsel have reviewed the issuance of the bonds,
related proceedings or the basis for the opinions of counsel for the issuers. We
cannot assure you that the issuers (or other users of bond proceeds) have
complied or will comply with any requirements necessary for a bond to be
tax-exempt. If any of the bonds were determined not to be tax-exempt, you could
be required to pay income tax for current and prior years, and if the Fund were
to sell the bond, it might have to sell it at a substantial discount.

In the opinion of our counsel, under existing law:

GENERAL TREATMENT OF THE FUND AND YOUR INVESTMENT

The Fund will not be taxed as a corporation for federal income tax purposes, and
you will be considered to own directly your share of each bond in the Fund.

GAIN OR LOSS UPON DISPOSITION

When all or part of your share of a bond is disposed of (for example, when the
Fund sells, exchanges or redeems a bond or when you sell or exchange your
units), you will generally recognize capital gain or loss. Your gain, however,
will generally be ordinary income to the extent of any accrued "market
discount". Generally you will have market discount to the extent that your basis
in a bond when you purchase a unit is less than its stated redemption price at
maturity (or, if it is an original issue discount bond, the issue price
increased by original issue discount that has accrued on the bond before your
purchase). You should consult your tax adviser in this regard.

                                       15
<PAGE>
If your net long-term capital gains exceed your net short-term capital losses,
the excess may be subject to tax at a lower rate than ordinary income. Any
capital gain from the Fund will be long-term if you are considered to have held
your investment on each bond for more than one year and short-term otherwise.
Because the deductibility of capital losses is subject to limitations, you may
not be able to deduct all of your capital losses.

YOUR BASIS IN THE BONDS

Your aggregate basis in the bonds will be equal to the cost of your units,
including any sales charges you pay, adjusted to reflect any accruals of
"original issue discount," "acquisition premium" and "bond premium". You should
consult your tax adviser in this regard.

EXPENSES

If you are not a corporate investor, you will not be entitled to a deduction for
your share of fees and expenses of the Fund. Also, if you borrowed money in
order to purchase or carry your units, you will not be able to deduct the
interest on this borrowing for federal income tax purposes. The IRS may treat
your purchase of units as made with borrowed money even if the money is not
directly traceable to the purchase of units.

NEW YORK TAXES

Under the income tax laws of the State and City of New York, the Fund will not
be taxed as a corporation. If you are a New York taxpayer, your income from the
Fund will not be tax-exempt in New York except to the extent that the income is
earned on bonds that are tax-exempt for New York purposes. Depending on where
you live, your income from the Fund may be subject to state and local taxation.
You should consult your tax adviser in this regard.

SUPPLEMENTAL INFORMATION

You can receive at no cost supplemental information about the Fund by calling
the Trustee. The supplemental information includes more detailed risk disclosure
about the types of bonds that may be in the Fund's portfolio, general risk
disclosure concerning any insurance securing certain bonds, and general
information about the structure and operation of the Fund. The supplemental
information is also available from the SEC.

                                       16
<PAGE>

          MUNICIPAL INVESTMENT TRUST FUND,
          INSURED SERIES - 401,
          DEFINED ASSET FUNDS

          REPORT OF INDEPENDENT ACCOUNTANTS

          The Sponsors, Trustee and Holders
          of Municipal Investment Trust Fund,
          Insured Series - 401,
          Defined Asset Funds:

          We have audited the accompanying statement of condition of Municipal
          Investment Trust Fund, Insured Series - 401, Defined Asset Funds,
          including the portfolio, as of October 31, 2000 and the related
          statements of operations and of changes in net assets for the year
          ended October 31, 2000 and the period November 21, 1998 to October 31,
          1999. These financial statements are the responsibility of the
          Trustee. Our responsibility is to express an opinion on these
          financial statements based on our audits.

          We conducted our audits in accordance with auditing standards
          generally accepted in the United States of America. Those standards
          require that we plan and perform the audit to obtain reasonable
          assurance about whether the financial statements are free of material
          misstatement. An audit includes examining, on a test basis, evidence
          supporting the amounts and disclosures in the financial statements.
          Securities owned at October 31, 2000, as shown in such portfolio, were
          confirmed to us by The Chase Manhattan Bank, the Trustee. An audit
          also includes assessing the accounting principles used and significant
          estimates made by the Trustee, as well as evaluating the overall
          financial statement presentation. We believe that our audits provide a
          reasonable basis for our opinion.

          In our opinion, the financial statements referred to above present
          fairly, in all material respects, the financial position of Municipal
          Investment Trust Fund, Insured Series - 401, Defined Asset Funds at
          October 31, 2000 and the results of its operations and changes in its
          net assets for the above-stated periods in conformity with accounting
          principles generally accepted in the United States of America.


          DELOITTE & TOUCHE LLP


          New York, N.Y.
          December 7, 2000


                                     D - 1.
<PAGE>

     MUNICIPAL INVESTMENT TRUST FUND,
     INSURED SERIES - 401,
     DEFINED ASSET FUNDS


     STATEMENT OF CONDITION
     As of October 31, 2000

<TABLE>
     <S>                                                                                <C>             <C>
     TRUST PROPERTY:
       Investment in marketable securities -
          at value (cost $ 8,295,702 )(Note 1).........                                                 $ 7,564,608
       Accrued interest ...............................                                                     151,530
       Accrued interest on Segregated Bonds (Note 5) ..                                                       1,805
       Cash - income ..................................                                                       9,342
       Cash - income on Segregated Bonds ..............                                                       5,718
       Cash - principal ...............................                                                       9,862
       Prepaid Trustee's fees and expense..............                                                         999
                                                                                                        -----------
         Total trust property .........................                                                   7,743,864


     LESS LIABILITIES:
       Deferred sales charge (Note 5) .................                                 $    30,641
       Principal payments payable (Segregated Bonds) ..                                       8,954
       Accrued Sponsors' fees .........................                                       3,469          43,064
                                                                                        -----------     -----------


     NET ASSETS, REPRESENTED BY:
       8,295,243 units of fractional undivided
          interest outstanding (Note 3)................                                   7,542,398

       Undistributed net investment income ............                                     158,402     $ 7,700,800
                                                                                        -----------     ===========

     UNIT VALUE ($ 7,700,800 / 8,295,243 units ).......                                                 $    .92834
                                                                                                        ===========
</TABLE>

                       See Notes to Financial Statements.


                                     D - 2.
<PAGE>

     MUNICIPAL INVESTMENT TRUST FUND,
     INSURED SERIES - 401,
     DEFINED ASSET FUNDS


     STATEMENTS OF OPERATIONS

<TABLE>
<CAPTION>
                                                                                                    November 21, 1998
                                                                                        Year Ended           to
                                                                                        October 31,      October 31,
                                                                                          2000              1999
                                                                                          ----              ----

     <S>                                                                              <C>               <C>
     INVESTMENT INCOME:
       Interest income ........................                                       $   446,008       $   473,507
       Interest income on Segregated
         Bonds (Note 5) .......................                                             4,494             6,318
       Trustee's fees and expenses ............                                            (9,662)           (5,457)
       Sponsors' fees .........................                                            (4,271)           (3,677)
                                                                                      ------------------------------
       Net investment income ..................                                           436,569           470,691
                                                                                      ------------------------------


     REALIZED AND UNREALIZED GAIN (LOSS)
       ON INVESTMENTS:
       Realized loss on
         securities sold or redeemed ..........                                          (200,168)          (14,879)
       Unrealized appreciation (depreciation)
         of investments .......................                                           615,783        (1,346,877)
                                                                                      ------------------------------
       Net realized and unrealized
          gain (loss) on investments ..........                                           415,615        (1,361,756)
                                                                                      ------------------------------


     NET INCREASE (DECREASE) IN NET ASSETS
       RESULTING FROM OPERATIONS ..............                                       $   852,184       $  (891,065)
                                                                                      ==============================
</TABLE>

                       See Notes to Financial Statements.


                                     D - 3.
<PAGE>

     MUNICIPAL INVESTMENT TRUST FUND,
     INSURED SERIES - 401
     DEFINED ASSET FUNDS


     STATEMENTS OF CHANGES IN NET ASSETS

<TABLE>
<CAPTION>
                                                                                                     November 21, 1998
                                                                                        Year Ended           to
                                                                                        October 31,       October 31,
                                                                                          2000              1999
                                                                                          ----              ----
     <S>                                                                              <C>               <C>
     OPERATIONS:
       Net investment income ..................                                       $   436,569       $   470,691
       Realized  loss  on
         securities sold or redeemed ..........                                          (200,168)          (14,879)
       Unrealized  appreciation (depreciation)
         of investments .......................                                           615,783        (1,346,877)
                                                                                      ------------------------------
       Net increase (decrease) in net assets
         resulting from operations ............                                           852,184          (891,065)
                                                                                      ------------------------------

     INCOME DISTRIBUTIONS TO
        HOLDERS (Note 2).......................                                          (445,643)         (272,177)
                                                                                      ------------------------------

     SHARE TRANSACTIONS (Note 5): Deferred sales charge (Note 5):
         Income ...............................                                            (3,289)
         Principal ............................                                           (97,486)          (49,906)

       Redemption amounts:
         Income................................                                           (17,633)           (2,593)
         Principal.............................                                        (1,331,815)         (150,208)
                                                                                         ---------------------------

     Total share transactions..................                                        (1,450,223)         (202,707)
                                                                                         ---------------------------

     NET DECREASE IN NET ASSETS ...............                                        (1,043,682)       (1,365,949)

     NET ASSETS AT BEGINNING OF PERIOD ........                                         8,744,482        10,110,431
                                                                                      ------------------------------
     NET ASSETS AT END OF PERIOD ..............                                       $ 7,700,800       $ 8,744,482
                                                                                      ==============================
     PER UNIT:
       Income distributions during
         period ...............................                                       $   0.04909       $   0.02738
                                                                                      ==============================
       Net asset value at end of
         period ...............................                                       $   0.92834       $   0.88884
                                                                                      ==============================
     TRUST UNITS:
       Redeemed during period .................                                         1,542,810           161,947
       Outstanding at end of period ...........                                         8,295,243         9,838,053
                                                                                      ==============================
</TABLE>

                       See Notes to Financial Statements.


                                     D - 4.
<PAGE>

          MUNICIPAL INVESTMENT TRUST FUND,
          INSURED SERIES - 401,
          DEFINED ASSET FUNDS

          NOTES TO FINANCIAL STATEMENTS

     1.   SIGNIFICANT ACCOUNTING POLICIES

          The Fund is registered under the Investment Company Act of 1940 as a
          Unit Investment Trust. The following is a summary of significant
          accounting policies consistently followed by the Fund in the
          preparation of its financial statements. The policies are in
          conformity with accounting principles generally accepted in the United
          States of America.

           (A)      Securities are stated at value as determined by the
                    Evaluator based on bid side evaluations for the securities,
                    except that value on November 21, 1998 was based upon
                    offering side evaluations at November 19, 1998, the day
                    prior to the Date of Deposit. Cost of securities at November
                    21, 1998 was also based on such offering side evaluations.

           (B)      The Fund is not subject to income taxes. Accordingly, no
                    provision for such taxes is required.

           (C)      Interest income is recorded as earned.

     2.   DISTRIBUTIONS

          Semi-annual distributions of net investment income are made to
          Holders. Receipts other than interest, after deductions for
          redemptions and applicable expenses, are also distributed
          periodically.

     3.   NET CAPITAL

<TABLE>
     <S>                                                                                                <C>
          Cost of 8,295,243 units at Date of Deposit .................                                  $ 8,386,849
          Transfer to capital of interest on Segregated Bonds (Note 5)                                       10,812
          Redemptions of units - net cost of 1,704,757 units redeemed
            less redemption amounts (principal).......................                                      241,559
          Deferred sales charge (Note 5) .............................                                     (150,681)
          Realized loss on securities sold or redeemed ...............                                     (215,047)
          Net unrealized depreciation of investments .................                                     (731,094)
                                                                                                        -----------

          Net capital applicable to Holders ..........................                                  $ 7,542,398
                                                                                                        ===========
</TABLE>

     4.   INCOME TAXES

          As of October 31, 2000, unrealized depreciation of investments, based
          on cost for Federal income tax purposes, aggregated $ 731,094, all of
          which related to depreciated securities. The cost of investment
          securities for Federal income tax purposes was $8,295,702 at October
          31, 2000.


                                     D - 5.
<PAGE>

          MUNICIPAL INVESTMENT TRUST FUND,
          INSURED SERIES - 401,
          DEFINED ASSET FUNDS

          NOTES TO FINANCIAL STATEMENTS


     5.   DEFERRED SALES CHARGE

          $95,000 face amount of the State of Hawaii, Certs. of Part. (Kapolei
          State Office Bldg.), Ser. 1998 A, have been segregated to fund the
          deferred sales charges. The sales charges are being paid for with the
          proceeds received and by periodic sales or maturity of these bonds, as
          well as principal proceeds received in conjunction with the
          disposition on the unsegregated bonds in the portfolio. A deferred
          sales charge of $2.38 per 1,000 Unit is charged on a quarterly basis,
          and paid to the Sponsors periodically by the Trustee on behalf of the
          Holders, up to an aggregate of $19 per 1,000 Unit over the first two
          years of the life of the Fund. Should a Holder redeem Units prior to
          the second anniversary of the Fund, the remaining balance of the
          deferred sales charge will be charged.


                                     D - 6.
<PAGE>

     MUNICIPAL INVESTMENT TRUST FUND,
     INSURED SERIES - 401,
     DEFINED ASSET FUNDS

     PORTFOLIO
     As of October 31, 2000

<TABLE>
<CAPTION>
                                             Rating of                                            Optional
     Portfolio No. and Title of                Issues       Face                                 Redemption
            Securities                        (1) (4)      Amount     Coupon      Maturities(3) Provisions(3)   Cost(2)    Value(2)
            ----------                       ---------  ----------- -----------   ------------- ------------  ----------  ---------

<S>                                          <C>        <C>         <C>           <C>          <C>          <C>         <C>
   1 Washington Conv. Ctr. Auth., Washington    AAA     $ 1,025,000     4.750 %      2028      10/01/08     $   970,224 $   879,901
     D.C., Sr. Lien Dedicated Tax Rev. Bonds,                                                  @  100.000
     Ser. 1998 (AMBAC Ins.)

   2 The Board of Educ. of the Twp. of Wall,    AAA         155,000     4.750        2021      07/15/08         152,873     138,435
     Monmouth Cnty., NJ, Sch. Bonds (FSA Ins.)                                                 @  100.000

   3 Illinois Hlth. Fac. Auth., Rev. Bonds      AAA       1,000,000     5.000        2024      11/15/08         978,630     892,190
     (Rush-Presbyterian-St. Luke's Med. Ctr.                                                   @  101.000
     Oblig. Grp.), Ser. 1998 A (MBIA Ins.)

   4 Iowa Fin. Auth., Hosp. Facs. Rev. Bonds    AAA       1,000,000     5.125        2028      07/01/08       1,002,110     905,130
     (Iowa Hlth. Sys.), Ser. 1998 A (MBIA Ins.)                                                @  102.000

   5 County of Grand Traverse, MI, Hosp. Fin.   Aaa(m)      190,000     4.750        2022      07/01/08         183,449     163,708
     Auth., Rev. and Rfdg. Bonds (Munson                                                       @  101.000
     Healthcare Oblig. Grp.), Ser. 1998 A
     (AMBAC Ins.)

   6 New Jersey Hlth. Care Fac. Fin. Auth.,     AAA         250,000     4.500        2028      07/01/08         236,310     206,213
     Rev. Bonds (CentraState Med. Ctr. Oblig.                                                  @  101.000
     Grp. Issue), Ser. 1998 (AMBAC Ins.)

   7 Wisconsin Hlth. and Educl. Fac. Auth.,     AAA       1,000,000     5.125        2023      07/01/08         996,470     908,130
     Rev. Bonds (Agnesian Hlthcare., Inc.),                                                    @  101.000
     Ser. 1998 (MBIA Ins.)

   8 Housing Auth. of the City of Bellingham,   Aaa(m)    1,000,000     5.200        2027      11/01/08       1,003,820     893,690
     WA, Hsg. Rfdg. Rev. Bonds (Pacific Rim                                                    @  100.000
     and Cascade Meadows Projs.), Ser. 1998
     (MBIA Ins.)

   9 State of Hawaii, Certs. of Part. (Kapolei  AAA          95,000     3.800        2001      None              95,219      94,490
     State Office Bldg.), Ser. 1998 A (AMBAC
     Ins.) (5)
</TABLE>


                                     D - 7.
<PAGE>

     MUNICIPAL INVESTMENT TRUST FUND,
     INSURED SERIES - 401,
     DEFINED ASSET FUNDS

     PORTFOLIO
     As of October 31, 2000

<TABLE>
<CAPTION>
                                             Rating of                                            Optional
     Portfolio No. and Title of                Issues       Face                                 Redemption
            Securities                        (1)  (4)      Amount    Coupon      Maturities(3) Provisions(3)    Cost(2)   Value(2)
            ----------                       ---------  ----------- -----------   ------------  ------------  ----------  ---------

<S>                                          <C>        <C>         <C>           <C>          <C>          <C>         <C>
  10 Washington Cnty./St. George Interlocal     AAA     $   250,000     5.125 %      2022      12/01/07     $   250,515 $   233,248
     Agy., UT, Lease Rev. Bonds, Ser. 1997 A                                                   @  101.000
     (AMBAC Ins.)

  11 New York State Energy Research and Dev.    AAA       1,000,000     5.150        2025      11/01/08       1,008,860     934,240
     Auth., Poll. Ctl. Rfdg. Rev. Bonds                                                        @  102.000
     (Niagara Mohawk Pwr. Corp. Proj.), Ser.
     1998 A (AMBAC Ins.)

  12 Intermountain Pwr. Agy., UT, Pwr. Supply   AAA         500,000     5.000        2021      07/01/06         496,620     461,200
     Rev. Rfdg. Bonds, Ser. 1996 D (FSA Ins.)                                                  @  102.000

  13 Public Util. Dist. No. 2, Grant Cnty, WA,  AAA         135,000     5.200        2023      01/01/09         135,558     126,990
     Elec. Sys. Rev. Bonds, Ser. 1998 G (MBIA                                                  @  101.000
     Ins.)

  14 City of Chicago, Il, Sales Tax Rev. Bonds, Aaa(m)      775,000     5.250        2028      07/01/08         785,044     727,043
     Ser. 1998 (Financial Guaranty Ins.)                                                       @  102.000

                                                          ---------                                           ---------   ---------
     TOTAL                                              $ 8,375,000                                         $ 8,295,702 $ 7,564,608
                                                          =========                                           =========   =========
</TABLE>

                             See Notes to Portfolio.


                                     D - 8.
<PAGE>

     MUNICIPAL INVESTMENT TRUST FUND,
     INSURED SERIES - 401,
     DEFINED ASSET FUNDS

     NOTES TO PORTFOLIO
     As of October 31, 2000

    (1)   The ratings of the bonds are by Standard & Poor's Ratings Group or by
          Moody's Investors Service, Inc. if followed by "(m)", or by Fitch
          Investors Service, Inc. if followed by "(f)"; "NR" indicates that this
          bond is not currently rated by any of the above-mentioned rating
          services. These ratings have been furnished by the Evaluator but not
          confirmed with the rating agencies.

    (2)   See Notes to Financial Statements.

    (3)   Optional redemption provisions, which may be exercised in whole or in
          part, are initially at prices of par plus a premium, then subsequently
          at prices declining to par. Certain securities may provide for
          redemption at par prior or in addition to any optional or mandatory
          redemption dates or maturity, for example, through the operation of a
          maintenance and replacement fund, if proceeds are not able to be used
          as contemplated, the project is condemned or sold or the project is
          destroyed and insurance proceeds are used to redeem the securities.
          Many of the securities are also subject to mandatory sinking fund
          redemption commencing on dates which may be prior to the date on which
          securities may be optionally redeemed. Sinking fund redemptions are at
          par and redeem only part of the issue. Some of the securities have
          mandatory sinking funds which contain optional provisions permitting
          the issuer to increase the principal amount of securities called on a
          mandatory redemption date. The sinking fund redemptions with optional
          provisions may, and optional refunding redemptions generally will,
          occur at times when the redeemed securities have an offering side
          evaluation which represents a premium over par. To the extent that the
          securities were acquired at a price higher than the redemption price,
          this will represent a loss of capital when compared with the Public
          Offering Price of the Units when acquired. Distributions will
          generally be reduced by the amount of the income which would otherwise
          have been paid with respect to redeemed securities and there will be
          distributed to Holders any principal amount and premium received on
          such redemption after satisfying any redemption requests for Units
          received by the Fund. The estimated current return may be affected by
          redemptions.

    (4)   Insured by AAA-rated insurance companies that guarantee timely
          payments of principal and interest on the bonds (but not Fund units or
          the market value of the bonds before they mature).

    (5) These bonds have been segregated to fund the deferred sales charges.


                                     D - 9.
<PAGE>
              Defined
            Asset Funds-Registered Trademark-

<TABLE>
<S>                                      <C>
HAVE QUESTIONS ?                         MUNICIPAL INVESTMENT TRUST FUND
Request the most recent free             INSURED SERIES--401
Information Supplement                   (A Unit Investment Trust)
that gives more details about            ---------------------------------------
the Fund, by calling:                    This Prospectus does not contain
The Chase Manhattan Bank                 complete information about the
1-800-323-1508                           investment company filed with the
                                         Securities and Exchange Commission in
                                         Washington, D.C. under the:
                                         - Securities Act of 1933 (file no.
                                         333-65309) and
                                         - Investment Company Act of 1940 (file
                                         no. 811-1777).
                                         TO OBTAIN COPIES AT PRESCRIBED RATES--
                                         WRITE: Public Reference Section of the
                                         Commission
                                         450 Fifth Street, N.W., Washington,
                                         D.C. 20549-6009
                                         CALL: 1-800-SEC-0330.
                                         VISIT: http://www.sec.gov.
                                         No person is authorized to give any
                                         information or representations about
                                         this Fund not contained in this
                                         Prospectus or the Information
                                         Supplement, and you should not rely on
                                         any other information.
                                         ---------------------------------------
                                         When units of this Fund are no longer
                                         available, this Prospectus may be used
                                         as a preliminary prospectus for a
                                         future series, but some of the
                                         information in this Prospectus will be
                                         changed for that series.
                                         UNITS OF ANY FUTURE SERIES MAY NOT BE
                                         SOLD NOR MAY OFFERS TO BUY BE ACCEPTED
                                         UNTIL THAT SERIES HAS BECOME EFFECTIVE
                                         WITH THE SECURITIES AND EXCHANGE
                                         COMMISSION. NO UNITS CAN BE SOLD IN ANY
                                         STATE WHERE A SALE WOULD BE ILLEGAL.
                                                                     32702--1/01
</TABLE>
<PAGE>
                        MUNICIPAL INVESTMENT TRUST FUND
                                 INSURED SERIES
                              DEFINED ASSET FUNDS
                       CONTENTS OF REGISTRATION STATEMENT

  This Post-Effective Amendment to the Registration Statement on Form S-6
comprises the following papers and documents:

  The facing sheet of Form S-6.

  The cross-reference sheet (incorporated by reference to the Cross-Reference
Sheet to the Registration Statement of Defined Asset Funds Municipal Insured
Series, 1933 Act File No. 33-54565).

    The Prospectus.

    The Signatures.

The following exhibits:

    1.1.1 -- Form of Standard Terms and Conditions of Trust Effective as of
            October 21, 1993 (incorporated by reference to Exhibit 1.1.1 to the
            Registration Statement of Municipal Investment Trust Fund,
            Multi-state Series--48, 1933 Act File No. 33-50247).

    1.11.1-- Merrill Lynch Code of Ethics (incorporated by reference to Exhibit
            1.11.1 to the Post Effective Amendment No. 8 to the Registration
            Statement of Municipal Investment Trust Fund, Insured Series 186,
            1933 Act File No. 33-49159).

    1.11.2-- Municipal Investment Trust Fund Code of Ethics (incorporated by
            reference to Exhibit 1.11.2 to the Post Effective Amendment No. 8 to
            the Registration Statement of Municipal Investment Trust Fund,
            Insured Series 186, 1933 Act File No. 33-49159).

    4.1  --Consent of the Evaluator.

    5.1  --Consent of independent accountants.

    9.1  -- Information Supplement (incorporated by reference to Exhibit 9.1 to
           Amendment No. 2 to the Registration Statement of Municipal Investment
           Trust Fund, Multistate Series--400, 1933 Act File No. 333-53537).

                                      R-1
<PAGE>
                        MUNICIPAL INVESTMENT TRUST FUND
                              INSURED SERIES--401
                              DEFINED ASSET FUNDS

                                   SIGNATURES

  PURSUANT TO THE REQUIREMENTS OF THE SECURITIES ACT OF 1933, THE REGISTRANT,
MUNICIPAL INVESTMENT TRUST FUND, INSURED SERIES--401, DEFINED ASSET FUNDS
CERTIFIES THAT IT MEETS ALL OF THE REQUIREMENTS FOR EFFECTIVENESS OF THIS
REGISTRATION STATEMENT PURSUANT TO RULE 485(B) UNDER THE SECURITIES ACT OF 1933
AND HAS DULY CAUSED THIS REGISTRATION STATEMENT OR AMENDMENT TO THE REGISTRATION
STATEMENT TO BE SIGNED ON ITS BEHALF BY THE UNDERSIGNED THEREUNTO DULY
AUTHORIZED IN THE CITY OF NEW YORK AND STATE OF NEW YORK ON THE 17TH DAY OF
JANUARY, 2001.

               SIGNATURES APPEAR ON PAGES R-3, R-4, R-5 AND R-6.

    A majority of the members of the Board of Directors of Merrill Lynch,
Pierce, Fenner & Smith Incorporated has signed this Registration Statement or
Amendment to the Registration Statement pursuant to Powers of Attorney
authorizing the person signing this Registration Statement or Amendment to the
Registration Statement to do so on behalf of such members.

    A majority of the members of the Board of Directors of Salomon Smith Barney
Inc. has signed this Registration Statement or Amendment to the Registration
Statement pursuant to Powers of Attorney authorizing the person signing this
Registration Statement or Amendment to the Registration Statement to do so on
behalf of such members.

    A majority of the members of the Executive Committee of the Board of
Directors of PaineWebber Incorporated has signed this Registration Statement or
Amendment to the Registration Statement pursuant to Powers of Attorney
authorizing the person signing this Registration Statement or Amendment to the
Registration Statement to do so on behalf of such members.

    A majority of the members of the Board of Directors of Dean Witter Reynolds
Inc. has signed this Registration Statement or Amendment to the Registration
Statement pursuant to Powers of Attorney authorizing the person signing this
Registration Statement or Amendment to the Registration Statement to do so on
behalf of such members.

                                      R-2
<PAGE>
               MERRILL LYNCH, PIERCE, FENNER & SMITH INCORPORATED
                                   DEPOSITOR

<TABLE>
<S>                                       <C>
By the following persons, who constitute  Powers of Attorney have been filed
  a majority of                             under
  the Board of Directors of Merrill         Form SE and the following 1933 Act
  Lynch, Pierce,                            File
  Fenner & Smith Incorporated:              Number: 333-70593
</TABLE>

     GEORGE A. SCHIEREN
     JOHN L. STEFFENS

     By JAY M. FIFE
       (As authorized signatory for Merrill Lynch, Pierce,
       Fenner & Smith Incorporated and
       Attorney-in-fact for the persons listed above)

                                      R-3
<PAGE>
                           SALOMON SMITH BARNEY INC.
                                   DEPOSITOR

<TABLE>
<S>                                                           <C>
By the following persons, who constitute a majority of        Powers of Attorney
  the Board of Directors of Salomon Smith Barney Inc.:          have been filed
                                                                under the 1933 Act
                                                                File Numbers:
                                                                333-63417 and
                                                                333-63033
</TABLE>

     MICHAEL A. CARPENTER
     DERYCK C. MAUGHAN

     By GINA LEMON
       (As authorized signatory for
       Salomon Smith Barney Inc. and
       Attorney-in-fact for the persons listed above)

                                      R-4
<PAGE>
                            PAINEWEBBER INCORPORATED
                                   DEPOSITOR

<TABLE>
<S>                                       <C>
By the following persons, who constitute  Powers of Attorney have been filed
  the Board of Directors of PaineWebber     under
  Incorporated:                             the following 1933 Act File
                                            Number: 33-55073
</TABLE>

     MARGO N. ALEXANDER
     TERRY L. ATKINSON
     BRIAN M. BAREFOOT
     STEVEN P. BAUM
     MICHAEL CULP
     REGINA A. DOLAN
     JOSEPH J. GRANO, JR.
     EDWARD M. KERSCHNER
     JAMES P. MacGILVRAY
     DONALD B. MARRON
     ROBERT H. SILVER
     MARK B. SUTTON

     By ROBERT E. HOLLEY
       (As authorized signatory for
       PaineWebber Incorporated
       and Attorney-in-fact for the persons listed above)

                                      R-5
<PAGE>
                           DEAN WITTER REYNOLDS INC.
                                   DEPOSITOR

<TABLE>
<S>                                       <C>
By the following persons, who constitute  Powers of Attorney have been filed
  a majority of                             under Form SE and the following 1933
  the Board of Directors of Dean Witter     Act File Numbers: 33-17085,
  Reynolds Inc.:                            333-13039, 333-47553, 333-89009 and
                                            333-39302.
</TABLE>

     BRUCE F. ALONSO
     RICHARD M. DeMARTINI
     RAYMOND J. DROP
     JAMES F. HIGGINS
     DONALD G. KEMPF, JR.
     JOHN J. MACK
     MITCHELL M. MERIN
     STEPHEN R. MILLER
     PHILIP J. PURCELL
     JOHN H. SCHAEFER
     THOMAS C. SCHNEIDER
     ALAN A. SCHRODER
     ROBERT G. SCOTT

     By MICHAEL D. BROWNE
       (As authorized signatory for
       Dean Witter Reynolds Inc.
       and Attorney-in-fact for the persons listed above)

                                      R-6



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