MUNICIPAL INVESTMENT TRUST FD INSURED SER 312 DEF ASSET FDS
497, 1999-09-17
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<PAGE>
                                     DEFINED ASSET FUNDSSM
- --------------------------------------------
- -----------------------------------


                              MUNICIPAL INVESTMENT TRUST FUND
                              INSURED SERIES--312
                              (A UNIT INVESTMENT TRUST)
                              O   PORTFOLIO OF INSURED LONG TERM MUNICIPAL BONDS
                              O   DESIGNED TO BE FREE OF REGULAR FEDERAL INCOME
                                  TAX
                              O   MONTHLY INCOME DISTRIBUTIONS



SPONSORS:                      -------------------------------------------------
Merrill Lynch,                 The Securities and Exchange Commission has not
Pierce, Fenner & Smith         approved or disapproved these Securities or
Incorporated                   passed upon the adequacy of this prospectus. Any
Salomon Smith Barney Inc.      representation to the contrary is a criminal
PaineWebber Incorporated       offense.
Dean Witter Reynolds Inc.      Prospectus dated September 17, 1999.


<PAGE>
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Defined Asset FundsSM
Defined Asset FundsSM 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:
   o A disciplined strategy of buying and holding with a long-term view is the
     cornerstone of Defined Asset Funds.
   o Fixed portfolio: Defined Funds follow a buy and hold investment strategy;
     funds are not managed and portfolio changes are limited.
o Defined Portfolios: We choose the stocks and bonds in advance, so you know
  what you're investing in.
o Professional research: Our dedicated research team seeks out stocks or bonds
      appropriate for a particular fund's objectives.
o 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, MAY
31, 1999.


CONTENTS
                                                              PAGE
                                                         ---------
Risk/Return Summary....................................          3
What You Can Expect From Your Investment...............          7
   Monthly Income......................................          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.......................................         13
   Auditors............................................         14
   Sponsors............................................         14
   Trustee.............................................         14
   Underwriters' and Sponsors' Profits.................         14
   Public Distribution.................................         14
   Code of Ethics......................................         14
   Year 2000 Issues....................................         15
Taxes..................................................         15
Supplemental Information...............................         16
Financial Statements...................................        D-1


                                       2
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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?
        O  The Fund plans to hold to maturity 11 insured long-term
           tax-exempt municipal bonds, and some short-term bonds
           reserved to pay the deferred sales charge, with a current
           aggregate face amount of $10,410,000.
        o  The Fund is a unit investment trust which means that,
           unlike a mutual fund, the Fund's portfolio is not managed.
        o  The bonds are rated AAA or Aaa by Standard & Poor's,
           Moody's or Fitch.
        o  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.
        o  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:



                                                 APPROXIMATE
                                                  PORTFOLIO
                                                  PERCENTAGE



o          General Obligation                               1%
        o  Hospital/Health Care                           40%
        o  Housing                                           4%
o          Industrial Development Revenue              15%
o          State/Local Government Supported          11%
        o  Municipal Electric Utilities                    10%
o          Universities/Colleges                            19%



       4.  WHAT ARE THE SIGNIFICANT RISKS?
           YOU CAN LOSE MONEY BY INVESTING IN THE FUND. THIS CAN
           HAPPEN FOR VARIOUS REASONS, INCLUDING:
        o  Rising interest rates, an issuer's worsening financial
           condition or a drop in bond ratings can reduce the price of
           your units.
        o  Because the Fund is concentrated in hospital/health care
           bonds, adverse developments in this sector may affect the
           value of your units.
        o  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.
        o  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 monthly 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


           WHAT YOU MAY EXPECT (PAYABLE ON THE 25TH DAY OF
           EACH MONTH):
           Regular Monthly Income per unit                   $    4.00
           Annual Income per unit                            $   48.06
           RECORD DAY: 10th day of each month
           These figures are estimates on the evaluation date; actual
           payments may vary.



       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.
           INVESTOR FEES
           Maximum Sales Fee (Load) on new
           purchases (as a percentage of
           $1,000 invested)                                  2.90%
           You will pay a deferred sales fee of $3.75 per unit
           quarterly November, February, May and August, through August
           2001.
           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:



                                         YOUR MAXIMUM
                                            SALES FEE
                 IF YOU INVEST:              WILL BE:
           ---------------------------  -----------------
           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%


           ESTIMATED ANNUAL FUND OPERATING EXPENSES


                                                        AMOUNT
                                                      PER UNIT
                                                    -----------
                                                     $    0.72
           Trustee's Fee
                                                     $    0.46
           Portfolio Supervision,
           Bookkeeping and
           Administrative Fees
           (including updating
           expenses)
                                                     $    0.13
           Organization Costs
                                                     $    0.20
           Evaluator's Fee
                                                     $    0.38
           Other Operating Expenses
                                                    -----------
                                                     $    1.89
           TOTAL



           The Sponsors historically paid organization costs and
           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 between January 5,
           1988 and October 17, 1996 and were outstanding on June 30,
           1999. 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 6/30/99.



                   WITH SALES FEE                    NO SALES FEE
            1 YEAR     5 YEARS   10 YEARS    1 YEAR     5 YEARS   10 YEARS


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


High         3.20%      6.96%      5.93%      4.14%      8.16%      6.52%
Average      -0.26      5.32       5.82       2.89       6.38       6.41
Low          -3.80      3.53       5.74       0.45       4.36       6.33


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


Average
Sales fee    3.23%      5.22%      5.82%


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

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>
           UNIT PRICE PER UNIT                       $979.80
           (as of May 31, 1998)
           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 monthly. 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
           $5.00 per unit. 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 monthly 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>
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    TAX-FREE VS. TAXABLE INCOME: A COMPARISON OF TAXABLE AND TAX-FREE YIELDS

<TABLE>
<CAPTION>

                                  EFFECTIVE
TAXABLE INCOME 1999*               % 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>      <C>     <C>
$      0- 25,750 $      0- 43,050   15.00     3.53   4.12     4.71   5.29     5.88   6.47     7.06    7.65
- ----------------------------------------------------------------------------------------------------------
$ 25,751- 62,450 $ 43,051-104,050   28.00     4.17   4.86     5.56   6.25     6.94   7.64     8.33    9.03
- ----------------------------------------------------------------------------------------------------------
$ 62,451-130,250 $104,051-158,550   31.00     4.35   5.07     5.80   6.52     7.25   7.97     8.70    9.42
- ----------------------------------------------------------------------------------------------------------
$130,251-283,150 $158,551-283,150   36.00     4.69   5.47     6.25   7.03     7.81   8.59     9.38   10.16
- ----------------------------------------------------------------------------------------------------------
OVER $283,151       OVER $283,151   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 1999 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 and local taxation. Consequently, investors are urged to consult their
own tax advisers in this regard.

                                       6
<PAGE>
WHAT YOU CAN EXPECT FROM YOUR INVESTMENT

MONTHLY INCOME

The Fund will pay you regular monthly income. Your monthly income may vary
because of:
   o elimination of one or more bonds from the Fund's portfolio because of
     calls, redemptions or sales;
   o a change in the Fund's expenses; or
   o the failure by a bond's issuer to pay interest.

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

Along with your monthly 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):


 Estimated Annual                  Estimated
 Interest Income        -       Annual Expenses
- -------------------------------------------------
                   Unit Price


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:
o a monthly statement of income payments and any principal payments;
o a notice from the Trustee when new bonds are deposited in exchange or
  substitution for bonds originally deposited;
o an annual report on Fund activity; and
o 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:
o copies of bond evaluations to enable you to comply with federal and state tax
  reporting requirements; and
o 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:
   o 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;
   o hospitals face increasing competition resulting from hospital mergers and
      affiliations;
   o hospitals need to reduce costs as HMOs increase market penetration and
     hospital supply and drug companies raise prices;
   o hospitals and health care providers are subject to various legal claims by
     patients and others and are adversely affected by increasing costs of
     insurance;
   o 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;
   o 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 providers may receive lower reimbursements and these would have
     to cut expenses to maintain profitability; and
   o most retirement/nursing home providers rely on entrance fees for operating
      revenues. If people live longer than

                                       8
<PAGE>
      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:
   o limiting real property taxes,
   o reducing tax rates,
   o imposing a flat or other form of tax, or
   o 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:
   o adding the value of the bonds, net accrued interest, cash and any other
     Fund assets;
   o 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
   o 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 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 25 years, but we
could discontinue it without prior notice for any business reason.

                                       9
<PAGE>
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:
   o if the New York Stock Exchange is closed (other than customary weekend and
      holiday closings);
   o 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
   o 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 certain other funds at a reduced sales fee if your investment goals change.
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.

                                       10
<PAGE>
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 monthly income. When you sell your units you will
receive your share of this cash.

In addition, as with mutual funds, the Fund (and therefore the investors) pay
all or some of the costs of organizing the Fund including:
   o cost of initial preparation of legal documents;
   o federal and state registration fees;
   o initial fees and expenses of the Trustee;
   o initial audit; and
   o legal expenses and other out-of-pocket expenses.

These costs are amortized over the first five years of the Fund.

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:
   o to reimburse the Trustee for the Fund's operating expenses;
   o for extraordinary services and costs of indemnifying the Trustee and the
      Sponsors;
   o costs of actions taken to protect the Fund and other legal fees and
     expenses;
   o expenses for keeping the Fund's registration statement current; and
   o 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 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

                                       11
<PAGE>
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.

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

                                       12
<PAGE>
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:
   o to cure ambiguities;
   o to correct or supplement any defective or inconsistent provision;
   o to make any amendment required by any governmental agency; or
   o 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:
   o it fails to perform its duties and the Sponsors determine that its
     replacement is in your best interest; or
   o 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:
   o remove it and appoint a replacement Sponsor;
   o liquidate the Fund; or
   o continue to act as Trustee without a Sponsor.

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

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.

                                       13
<PAGE>
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.

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

Merrill Lynch, as agent for the Sponsors, has adopted a code of ethics requiring
preclearance and reporting of personal securities transactions by its employees
with access to information on portfolio

                                       14
<PAGE>
transactions. The goal of the code is to prevent fraud, deception or misconduct
against the Fund and to provide reasonable standards of conduct.

YEAR 2000 ISSUES

Many computer systems were designed in such a way that they may be unable to
distinguish between the year 2000 and the year 1900 (commonly known as the 'Year
2000 Problem'). We do not expect that the computer system changes necessary to
prepare for the Year 2000 will cause any major operational difficulties for the
Fund. The Year 2000 Problem may adversely affect the issuers of the bonds
contained in the Portfolio, but we cannot predict whether any impact will be
material to the Fund as a whole.

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. 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 federal
alternative minimum tax. 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 issuer (or other users) 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.

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. If
you are

                                       15
<PAGE>
an individual and sell your units after holding them for more than one year, you
may be entitled to a 20% maximum federal tax rate on any resulting gains.
Consult your tax adviser in this regard. 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 and the organizational expenses 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.

STATE AND LOCAL 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 - 312,
          DEFINED ASSET FUNDS

          REPORT OF INDEPENDENT ACCOUNTANTS

          The Sponsors, Trustee and Holders
          of Municipal Investment Trust Fund,
          Insured Series - 312,
          Defined Asset Funds:

          We have audited the accompanying statement of condition of
          Municipal Investment Trust Fund, Insured Series - 312,
          Defined Asset Funds, including the portfolio, as of
          May 31, 1999 and the related statements of operations
          and of changes in net assets for the period June 5, 1998 to
          May 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 audit.

          We conducted our audit in accordance with generally
          accepted auditing standards. 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 May 31,
          1999, 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 audit provides 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 - 312, Defined Asset Funds at May 31, 1999
          and the results of its operations and changes in its net
          assets for the above-stated period in conformity with
          generally accepted accounting principles.



          DELOITTE & TOUCHE LLP



          New York, N.Y.
          August 10, 1999
                                                    D - 1.
<PAGE>
     MUNICIPAL INVESTMENT TRUST FUND,
     INSURED SERIES - 312,
     DEFINED ASSET FUNDS



     STATEMENT OF CONDITION
     As of May 31, 1999

<TABLE>
     <S>                                                                                <C>             <C>
     TRUST PROPERTY:
       Investment in marketable securities -
{ 20}     at value (cost $ 10,182,081 )(Note 1)........                                                 $ 9,914,839
{ 36}  Accrued interest ...............................                                                     168,275
       Accrued interest on Segregated Bonds (Note 5) ..                                                       4,781
       Cash - income on Segregated Bonds ..............                                                      14,151
{ 34}  Cash - principal ...............................                                                      85,470
       Income payments receivable .....................                                                       2,470
       Deferred organization costs (Note 6) ...........                                                       8,352
                                                                                                        -----------
{ 40}    Total trust property .........................                                                  10,198,338


     LESS LIABILITIES:
{ 50}  Income advance from Trustee.....................                                 $   140,613
       Deferred sales charge (Note 5) .................                                     113,434
       Principal payments payable .....................                                       2,470
       Accrued Sponsors' fees .........................                                       1,938
       Other liabilities (Note 6) .....................                                       8,352         266,807
                                                                                        -----------     -----------


     NET ASSETS, REPRESENTED BY:
{ 80}  10,083 units of fractional undivided
{ 80}     interest outstanding (Note 3)................                                   9,903,337

{105}  Undistributed net investment income ............                                      28,194     $ 9,931,531
                                                                                        -----------     ===========

{130}UNIT VALUE ($ 9,931,531 / 10,083 units ).........                                                 $    984.98
                                                                                                        ===========


</TABLE>


                         See Notes to Financial Statements.
                                         D - 2.

<PAGE>
     MUNICIPAL INVESTMENT TRUST FUND,
     INSURED SERIES - 312,
     DEFINED ASSET FUNDS



     STATEMENT OF OPERATIONS
<TABLE>
<CAPTION>
                                                                                                       June 5, 1998
                                                                                                             to
                                                                                                           May 31,
                                                                                                            1999
                                                                                                            ----

     <S>                                                                                                <C>
     INVESTMENT INCOME:
{ 10}  Interest income ........................                                                         $   497,279
       Interest income on Segregated
         Bonds (Note 5) .......................                                                              18,932
{ 20}  Trustee's fees and expenses ............                                                             (12,149)
{ 30}  Sponsors' fees .........................                                                              (4,613)
                                                                                                        ------------
{ 40}  Net investment income ..................                                                             499,449
                                                                                                        ------------


       UNREALIZED DEPRECIATION
{ 60}    OF INVESTMENTS .......................                                                            (267,242)
                                                                                                        ------------


     NET INCREASE IN NET ASSETS
{ 80}  RESULTING FROM OPERATIONS ..............                                                         $   232,207
                                                                                                        ============


</TABLE>


                              See Notes to Financial Statements.
                                                  D - 3.
<PAGE>
     MUNICIPAL INVESTMENT TRUST FUND,
     INSURED SERIES - 312,
     DEFINED ASSET FUNDS



     STATEMENT OF CHANGES IN NET ASSETS
<TABLE>
<CAPTION>
                                                                                                       June 5, 1998
                                                                                                             to
                                                                                                           May 31,
                                                                                                            1999
                                                                                                            ----

     <S>                                                                                                <C>
     OPERATIONS:
{ 10}  Net investment income ..................                                                         $   499,449
       Unrealized depreciation
{ 30}    of investments .......................                                                            (267,242)
                                                                                                        ------------
       Net increase in net assets
{ 40}    resulting from operations ............                                                             232,207
                                                                                                        ------------

     INCOME DISTRIBUTIONS TO
{ 50}   HOLDERS (Note 2).......................                                                            (452,323)
                                                                                                        ------------

     SHARE TRANSACTIONS:
       Deferred sales charge (Note 5):
         Principal ............................                                                            (113,434)
                                                                                                        ------------

{ 90}NET DECREASE IN NET ASSETS ...............                                                            (333,550)

{100}NET ASSETS AT BEGINNING OF PERIOD ........                                                          10,265,081
                                                                                                        ------------
{110}NET ASSETS AT END OF PERIOD ..............                                                         $ 9,931,531
                                                                                                        ============
     PER UNIT:
       Income distributions during
{120}    period ...............................                                                         $     44.86
                                                                                                        ============
       Net asset value at end of
{140}    period ...............................                                                         $    984.98
                                                                                                        ============
     TRUST UNITS:
{150}  Outstanding at end of period ...........                                                              10,083
                                                                                                        ============
</TABLE>


                                 See Notes to Financial Statements.



                                           D - 4.

<PAGE>
          MUNICIPAL INVESTMENT TRUST FUND,
          INSURED SERIES - 312,
          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 generally accepted accounting principles.

           (A)      Securities are stated at value as determined by the
                    Evaluator based on bid side evaluations for the securities,
                    except that value on June 5, 1998 was based upon offering
                    side evaluations at June 3, 1998, the day prior to the
                    Date of Deposit. Cost of securities at June 5, 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

          A distribution of net investment income is made to Holders each month.
          Receipts other than interest, after deductions for redemptions and
          applicable expenses, are also distributed periodically.

     3.   NET CAPITAL
<TABLE>
     <S>                                                                                                <C>
{ 10}     Cost of 10,083 units at Date of Deposit ....................                                  $10,265,081
          Transfer to capital of interest on Segregated Bonds (Note 5)                                       18,932
          Deferred sales charge (Note 5) .............................                                     (113,434)
{ 70}     Net unrealized depreciation of investments..................                                     (267,242)
                                                                                                        -----------

{ 80}     Net capital applicable to Holders ..........................                                  $ 9,903,337
                                                                                                        ===========
</TABLE>
     4.   INCOME TAXES

          As of May 31, 1999, net unrealized depreciation of investments, based
          on cost for Federal income tax purposes, aggregated $267,242, of which
          $267,488 related to depreciated securities and $246 related to
          appreciated securities. The cost of investment securities for Federal
          income tax purposes was $10,182,081 at May 31, 1999.

                                             D - 5.
<PAGE>
MUNICIPAL INVESTMENT TRUST FUND,
INSURED SERIES - 312,
DEFINED ASSET FUNDS

NOTES TO FINANCIAL STATEMENTS


5. DEFERRED SALES CHARGE

$140,000 face amount of California Statewide Communities Dev. Auth., Sutter
Hlth. Oblig. Group, $140,000 face amount of New Jersey Educl. Fac. Auth., Higher
Educ. Fac. Trust Fund Bonds, Ser. 1995 A and $130,000 face amount of the County
of Pike, Commonwealth of Pennsylvania, G.O. Bonds, Ser. 1998 have been
segregated to fund the deferred sales charges. The sales charges are being paid
for withn 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 $3.75 per 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 $45
per Unit over the first three years of the life of the Fund. Should a Holder
redeem Units prior to the third anniversary of the Fund, the remaining balance
of the deferred sales charge will be charged.

6. DEFERRED ORGANIZATION COSTS

Deferred organization costs are being amortized over five years. Included in
"Other liabilities" is $8,352 payable to the Trustee for reimbursement of costs
related to the organization of the Trust.
































                                       D - 6.

<PAGE>
     MUNICIPAL INVESTMENT TRUST FUND,
     INSURED SERIES - 312,
     DEFINED ASSET FUNDS

     PORTFOLIO
     As of May 31, 1999

<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 California Hsg. Fin. Agy., Multifamily     AAA     $   400,000     5.300 %      2028      08/01/08     $   403,540 $   401,308
     Hsg. Rev. Bonds III, Ser. 1998 C                                                          @  101.500
     (MBIA Ins.)

   2 California Statewide Communities Dev.      AAA         140,000     5.250        2000      None             143,938     143,164
     Auth., Sutter Hlth. Oblig. Group
     (AMBAC Ins.) (5)

   3 Illinois Hlth. Fac. Auth., Rev. Rfdg.      Aaa(m)    1,000,000     5.125        2028      05/15/08         988,560     952,570
     Bonds (Northwestern Med. Faculty                                                          @  101.000
     Foundation, Inc.), Ser. 1998 (MBIA Ins.)

   4 Illinois Hlth. Facs. Auth., Rev. Bonds     AAA       1,000,000     5.250        2028      07/01/08       1,000,000     965,270
     (The Carle Foundation), Ser. 1998 A                                                       @  101.000
     (AMBAC Ins.)

   5 Indiana Educl. Fac. Auth., Educl. Fac.     AAA       1,000,000     5.000        2028      04/01/08         977,190     944,570
     Rev. Bonds (St. Mary's College Proj.),                                                    @  101.000
     Ser. 1998 (MBIA Ins.)

   6 Hosp. Auth. of St. Joseph Cnty., IN,       AAA       1,000,000     4.625        2028      02/15/08         915,650     873,440
     Hlth. Sys. Bonds (Memorial Hlth. Sys.),                                                   @  101.000
     Ser. 1998 A (MBIA Ins.)

   7 Massachusetts Bay Trans. Auth., Gen.       AAA       1,000,000     4.500        2026      03/01/08         911,370     878,440
     Trans. Sys. Bonds, Ser. 1998 A                                                            @  100.000
     (MBIA Ins.)

   8 New Jersey Educl. Fac. Auth., Higher       AAA         140,000     5.125        2001      None             144,739     144,185
     Educ. Fac. Trust Fund Bonds, Ser. 1995 A
     (AMBAC Ins.) (5)

   9 Long Island Pwr. Auth., NY, Elec. Sys.     AAA         600,000     5.125        2022      06/01/08         592,164     587,556
     Gen. Rev. Bonds, Ser. 1998 A (FSA Ins.)                                                   @  101.000


</TABLE>

                                                     D - 7.
<PAGE>

     MUNICIPAL INVESTMENT TRUST FUND,
     INSURED SERIES - 312,
     DEFINED ASSET FUNDS

     PORTFOLIO
     As of May 31, 1999

<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 Oliver Cnty., ND, Poll. Ctl. Rev.          AAA     $ 1,000,000     5.300 %      2027      01/01/09     $ 1,009,100 $   998,510
     Rfdg. Bonds (Square Butte Elec. Coop.),                                                   @  102.000
     Ser. 1998 A (AMBAC Ins.)

  11 City of Philadelphia. PA, Gas Works Rev.   AAA       1,000,000     5.000        2028      07/01/08         977,140     955,710
     Bonds (Gen. Ordinance), First Ser. 1998                                                   @  100.000
     B (FSA Ins.)

  12 County of Pike, Commonwealth of            Aaa(m)      130,000     3.800        1999      None             130,000     130,246
     Pennsylvania, G.O. Bonds, Ser. 1998
     (AMBAC Ins.) (5)

  13 Richland Cnty., SC, Educl. Facs. Rev.      AAA       1,000,000     5.125        2028      01/01/08         988,690     972,900
     Bonds, (Benedict Coll. Proj.), Ser.                                                       @  102.000
     1998 (FSA Ins.)

  14 Wisconsin Hlth. and Educl. Facs. Auth.,    AAA       1,000,000     5.250        2027      06/15/08       1,000,000     966,970
     Rev. Bonds (United Hlth. Grp. Inc.),                                                      @  101.000
     Ser. 1998 (AMBAC Ins.)

                                                        -----------                                          ----------  ----------
     TOTAL                                              $10,410,000                                         $10,182,081 $ 9,914,839
                                                        ===========                                          ==========  ==========

</TABLE>


                                               See Notes to Portfolio.






                                                       D - 8.

<PAGE>
MUNICIPAL INVESTMENT TRUST FUND,
INSURED SERIES - 312,
DEFINED ASSET FUNDS

NOTES TO PORTFOLIO
As of May 31, 1999

(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) All securities are insured, either on an individual basis or by portfolio
insurance, by a municipal bond insurance company which has been assigned
"AAA" claims paying ability by Standard & Poor's. Accordingly, Standard &
Poor's has assigned a "AAA" rating to the securities. Securities covered by
portfolio insurance are rated "AAA" only as long as they remain in the Trust.

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



                                  D - 9.


<PAGE>
                             Defined
                             Asset FundsSM


HAVE QUESTIONS ?                         MUNICIPAL INVESTMENT TRUST FUND
Request the most recent free             INSURED SERIES--312
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:
                                         o Securities Act of 1933 (file no.
                                         333-44071) and
                                         o 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.


                                                      70087--9/99


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