The mutagenic activities of benzidine, its dihydrochloride salt, and 12 of their analogues were compared in the Ames test using strains TA100 and TA98 with and without rat liver S9 activation. With the exceptions of 4,4'-methylenebis(3-nitroaniline) in both strains and 3,3-dichlorobenzidine in TA98, little or no mutagenicity was observed in the series when tested without S9 activation. All compounds, except tetramethylbenzidine, exhibited some activity in TA100 with S9 activation; dichlorobenzidine and 4-aminobiphenyl were significantly more mutagenic than the other compounds. This was in contrast to the TA98 results where the bridged diphenyl compounds, with the exception of the nitroaniline derivative, were only slightly mutagenic compared to the more planar biphenyl series. Only the nitroaniline compound was mutagenic in both strains in the presence or absence of S9 activation. For benzidine and the 3,3'-disubstituted benzidines (the dimethoxy-, diamino-, and dichloro- compounds), an increase in mutagenicity correlated to a decrease in basicity of the parent anilines in both TA100 and TA98.