The mutagenic potentials of the azo dyes, direct-black-38 (1937377), direct-blue-15 (2429745), and direct-red-2 (992596), and their aromatic amine reduction products were tested employing the Ames test and the arabinose resistant assay of Salmonella-typhimurium. S9 fractions were prepared from the livers of Sprague-Dawley-rats and Syrian-golden-hamsters, and S-typhimurium strains (TA-98) and (SV-50) were used in the mutagenicity tests. All three azo dyes tested were mutagenic in both the Ames and arabinose resistant assays; however the Ames test yielded relatively higher mutagenic responses than the arabinose resistant assay. Relatively higher mutagenic responses were observed with hamster S9 than with rat S9 in both assays. Direct-black-38 produced higher mutagenic responses in both assays than the other two dyes. The reduction products, benzidine (92875), o-dianisidine (119904), and o-tolidine (119937) were all mutagenic in the Ames test. In the arabinose resistant assay, benzidine was only weakly mutagenic, and o-dianisidine and o- tolidine were nonmutagenic. The authors conclude that both the Ames test and arabinose resistant assay are sensitive in testing the mutagenicity of azo dyes, but the Ames test is more efficient than the arabinose resistant assay in detecting mutagenicity of the aromatic amine reduction products of these dyes.