The antimutagenic and antioxidant enzyme activities of captopril (CAP), cysteine (CYS), and glutathione (GSH) were evaluated for finding concentration-dependent inhibitory effects against: (1) the mutagenicity of 2-amino-3-methylimidazo [4,5-f] quinoline (IQ), an indirect mutagen; and (2) N-methyl-N′-nitrosoguanidine (MNNG), a direct mutagen toward Salmonella typhimurium TA98 and TA100. Of the three thiols, CYS and GSH exhibited better inhibitory effects against IQ-induced mutation toward TA98 and TA100, respectively. GSH also showed a protective effect against MNNG-induced mutation toward TA98 and TA100, meanwhile, CAP showed the least inhibitory effect. CYS, GSH, and CAP also dose-dependently increased the activities of glutathione transferase, glutathione peroxidase, and glutathione reductase in hepatic BNL cells. CAP showed the superior inducing effects on glutathione transferase activity. These data suggested that the bioactive properties of biological thiols might contribute to their effects of antimutagenic activities as well as regulation on activities of antioxidant enzymes.