More complex environmental mixtures have been evaluated for mutagenic activity at the hisD3052 allele of Salmonella, primarily in strain TA98, than in any other target or mutation assay. Using colony probe hybridization to detect a common hot spot deletion, followed by polymerase chain reaction and DNA sequencing, we have generated 10 mutation spectra from three classes of mixtures (i.e., urban air, cigarette smoke condensate, and municipal waste incinerator emissions). The mutation spectra are distinctly different among the three classes of mixtures; however, the spectra for samples within the same class of mixture are similar. In addition to the hot spot mutation, the mixtures induce complex mutations, which consist of a small deletion and a base substitution. These mutations suggest a mechanism involving misinsertion of a base opposite a DNA adduct followed by a slippage and mismatch. A role for DNA secondary structure also may be the basis for the mutational site specificity exhibited by the various mixtures. The results suggest that unique mutation spectra can be generated by different classes of complex mixtures and that such spectra are a consequence of the dominance of a particular chemical class or classes within the mixture. The problems associated with this type of research are discussed along with the potential value of mutation spectra as a tool for exposure and risk assessment.