A commercial sample of the tuberculostatic drug isoniazid (INH) was found to have a weak mutagenic activity towards Salmonella typhimurium strains TA100 and TA1535. The addition of a rat or mouse liver homogenate to the test system decreased the mutagenic effect of INH. Hydrazine, an impurity of the INH sample, was also weakly mutagenic in strains TA100 and TA1535, but not in the extent that could account for the mutagenicity of the INH sample. An inhibition of DNA synthesis in human fibroblasts was observed for INH, this effect being potentiated by the addition of manganese to the test system. There was no induction of unscheduled DNA synthesis in human cells by INH except in the presence of manganese.