Carbadox and olaquindox were examined for mutagenicities in the repair tests with Bacillus subtilis (rec assay) and Salmonella typhimurium (uvr assay) and in the reverse mutation test (TA100 and TA98 of S. typhimurium). Both compounds were positive in the rec and uvr assays, and were highly mutagenic for strains TA100 and TA98. Carbadox was about 6 times move mutagenic than olaquindox in the absence of S9 mix. When incubated in S9 mix or bacterial cytosol (BC) mix for various times at 37 degree C, carbadox was found to lose its mutagenic activities easier than olaquindox. The mutagenicity of carbadox was almost inactivated at 10 min after incubation with S9 mix, but olaquindox still retained its activities even at 20 min. While carbadox required 20 min to be inactivated in BC mix, olaquindox was not completely inactivated even if incubated for 60 min.