Disclosed is a slime-removing composition comprising polyhexamethyleneguanidine phosphate and 2-bromo-2-nitro-1, 3-propanediol, represented by the following formulas I and II, respectively. With a synergistic effect, the composition has potent sterilizing activity against a broad spectrum of microorganisms which cause the formation of slime during paper manufacture, including bacteria and fungi. In addition to being excellent in rapid action and persistency of the sterilizing activity, the composition generates resistant mutants at an extremely low frequency, so that there can be obtained an economical favor of maintaining high antimicrobial activity even at a small amount. Also, by virtue of the availability in various phases, numerous applications of the microbicide exist in the various industries including cooling water and sterilizer industries in addition to paper and pulp making industries. ##STR1## wherein m is an integer of 4 to 7 and n is an integer of 1 to 14.
