Nitro derivatives of the heterocyclic compounds carbazole and 2-hydroxycarbazole were synthesized and tested in the Ames plate-incorporation assay in order to evaluate the contribution of these compounds to the mutagenicity of hazardous pollutants in ambient air and cigarette smoke. The results obtained in Salmonella typhimurium strain TA 98 found 3,6-dinitrocarbazole to be the most mutagenic derivative, followed by 2-hydroxy-l,3,6-trinitrocarbazole and 2-hydroxy-3-nitrocarbazole. 2-hydroxy-l,3-dinitrocarbazole was moderately active, and 1-nitrocarbazole weakly active, with 1,6-dinitrocarbazole and 3-nitrocarbazole ranging from weakly to moderately active. Carbazole, 2-hydroxycarbazole and 2-hydroxy-l-nitrocarbazole were completely non-mutagenic. These varying activities appear to derive from the different structures of the compounds, mainly the orientation of the nitro group, the presence of the hydroxy group, and the extent of nitro-substitution, as well as from the degree of nitroreduction by the "classical" nitroreductase in Salmonella and by the nitroreductases in the S9 mix, with the oxidases in S9 also possibly playing a role.