We previously observed that postmitochondrial supernatant (S9) from rat liver and kidney homogenates transforms L-cysteine into a mutagen that reverts bacteria of the strain Salmonella typhimurium TA100 to histidine independence. In the present study the enantiomers of cysteine and penicillamine (beta, beta-dimethylcysteine) have been investigated for mutagenicity. The Salmonella typhimurium strain TA92 was found to be more sensitive than TA100 to the mutagenic action of L-cysteine and was therefore also included. This strain allowed the unambiguous realization of a (weak) mutagenic effect of L-cysteine even in the absence of mammalian enzyme preparations. D-cysteine did not show mutagenicity under any experimental conditions. However, it was strongly bacteriotoxic. On the other hand, both enantiomers of penicillamine exerted clear mutagenic effects. Qualitatively, their mutagenicity was similar to that of L-cysteine in the following respects: the penicillamines were directly mutagenic, their mutagenicity was enhanced by S9, kidney S9 enhanced the mutagenicity more than did liver S9, TA92 was more sensitive than TA100. Thereby it is noteworthy that the ratios of the specific mutagenicities in the two strains were virtually identical in the direct, kidney-S9-mediated and liver-S9-mediated tests suggesting that the ultimate mutagens under these different metabolic conditions were identical. On the other hand, substantial quantitative differences in the mutagenicity between the beta-thiol amino acids were observed. L-penicillamine was about eight times more mutagenic than the clinically used enantiomer, D-penicillamine. In the direct tests, the mutagenic potency of L-cysteine was equal to that of D-penicillamine. In the S9-mediated experiments, the mutagenic potency of L-cysteine was intermediate between those of L- and D-penicillamine.