A radiochemical study for antimicrobial activity of chlormerodrin was performed using some commonly occurring nonpathogenic and pathogenic microorganisms. Chlormerodrin concentrations of 15-35 and 20-45 microgram/ml of the culture medium were microbiostatic and microbicidal, respectively. The microbiostatic effect was reversible in the presence of cysteine, an amino acid containing a sulfhydryl group.