Mutagenicity is frequently considered as a screening test for the carcinogenicity of compounds to which humans are exposed. Past estimates of the mutagenicity of ambient aerosol have depended on measurements of the mutagenic potency (refertants/microg) of the extractable organic matter (EOM) in Salmonella typhimurium from specific source emissions and their mass emission rate. This source-oriented approach requires an explicit model for the dispersion and possible transformation of the mutagens in the atmosphere. However, a receptor-oriented approach, demonstrated here, requires only ambient measurements of mutagenicity and of trace elements that are markers for the suspected sources of the mutagens. During winter at a residential site, woodsmoke and motor vehicle emissions were found to make nearly equal contributions to the average mutagenic concentration (refertants/cu m) of the fine particle ambient aerosol. Mutagenic potency of the fine particle EOM traced to motor vehicles, however, was three times greater than that with a woodsmoke origin. The results are consistent with the known potency of these sources, from source measurements.