Covalent modification of sulfhydryl groups which become sensitive toward sulfhydryl agents during germination of Bacillus cereus spores exerts a profound bacteriostatic effect, resulting in outgrowth inhibition. The modified spore components are membrane species of 13,000, 28,000, and 29,000 daltons. Detergent disruption of the membrane inactivated the sulfhydryl groups. A highly sigmoid inhibition curve (n = 11.8) with diamide suggested the participation of closely neighboring sulfhydryl groups. Substate and substrate analogs of the lactose and dicarboxylic acid permeases protected the sulfhydryl groups against modification. Nisin, a 34-residue peptide antibiotic, inhibited spore outgrowth and sulfhydryl modification at a concentration of about 0.1 microM. Since these sulfhydryl groups have been implicated as involved with the bacteriostatic action of nitrite, substances directed toward them may be a useful new class of bacteriostatic agents and antibiotics.