The mutagenic activity of all 4 isomeric aminocarbazoles and 4 nitrocarbazoles was evaluated in Salmonella typhimurium tester strains TA98. TA100 and TA1535. All compounds were assayed both in the presence and absence of liver homogenate from Aroclor-treated rats. Among the aminocarbazoles, 2-aminocarbazole was found to be most active in both tester strains, although somewhat less active than 2-aminofluorene. 3-Aminocarbazole was the only other isomer that was mutagenic towards TA98 at the dose levels employed (5--200 micrograms). 4-Aminocarbazole was moderately active in TA100, and 1-aminocarbazole was inactive in both TA98 and TA100. Similar differences in mutagenic potency and specificity towards the tester strains were observed for the related series of nitrocarbazoles.