<PAGE>
PROSPECTUS SUPPLEMENT DATED DECEMBER 11, 1998
TO PROSPECTUS DATED OCTOBER 14, 1998
741,876 SHARES
GALAGEN INC.
COMMON STOCK
-------------------
THIS PROSPECTUS SUPPLEMENT SHOULD BE READ IN CONJUNCTION WITH THE
PROSPECTUS DATED OCTOBER 14, 1998 (THE "PROSPECTUS"). THE SELLING STOCKHOLDERS
TABLE ON PAGE 4 OF THE PROSPECTUS IS HEREBY SUPPLEMENTED TO INCLUDE AN
ADDITIONAL SELLING STOCKHOLDER AS INDICATED IN THE FOLLOWING TABLE. AS USED
HEREIN, "SELLING STOCKHOLDER" INCLUDES DONEES AND PLEDGEES SELLING SHARES
RECEIVED FROM A NAMED SELLING STOCKHOLDER AFTER THE DATE OF THIS PROSPECTUS
SUPPLEMENT (PERCENTAGES ARE BASED ON 8,341,619 SHARES OUTSTANDING ON DECEMBER 7,
1998).
<TABLE>
<CAPTION>
SHARES OWNED SHARES OWNED
PRIOR TO OFFERING (1) AFTER OFFERING (2)
--------------------- -------------------
PERCENT OF SHARES PERCENT OF
NAME NUMBER OUTSTANDING OFFERED NUMBER OUTSTANDING
- ----------------------- ------ ----------- ------- ------ -----------
<S> <C> <C> <C> <C> <C>
H. David Cowherd 65,000 * 65,000 0 0
</TABLE>
- -------------------
* Less than 1%.
(1) Holdings are as of December 7, 1998 and assume full exercise of the
Warrants held by such Selling Stockholder.
(2) Assumes sale of all shares of the Selling Stockholder offered hereby.
