Cobalt(II) chloride (CoCl2), non-mutagenic by itself, has been tested for mutagenic activity in the presence of 4-substituted pyridines in the test strains of Salmonella typhimurium. CoCl2 was found to be mutagenic in strains TA1537 and TA2637, when combined pyridine, with methyl isonicotinate, 4-methyl-pyridine, 4-ethylpyridine, 4-chloropyridine or 4-bromopyridine. Mixtures of CoCl2 and isonicotinic acid, 4-cyanopyridine, 4-aminopyridine, or 4-dimethylaminopyridine exhibited no mutagenicity. Judging from the spectral observations, such combined mutagenicity may be due to the formation of moderate to weak complexes between these compounds and the Co(II) cation.