The mutagenic activity of complex environmental mixtures was evaluated in a newly developed bacterial tester strain. Extracts of distilled water, drinking water, swimming pool water, oil shale, diesel exhaust particulates, and nitrosated coal dust samples were tested in the Ames/Salmonella assay using strain (SV-50) with or without S9 mix. SV50 is a new, sensitive tester strain carrying an R-factor plasmid (pKM101) that had been shown previously to be capable of detecting frameshift and base pair mutations induced by a variety of known chemical mutagens. Results obtained with the diesel emission particulate and nitrosated coal dust extracts were compared with those using strain (TA-98). Both swimming pool and drinking water extracts were mutagenic in (SV-50). Based on water volume, the swimming pool water extract was more mutagenic. Adding S9 decreased the mutagenic activity of both extracts. No mutagenic activity was found in the distilled water extracts with or without S9 activation. The extracts from the other types of samples showed a dose dependent mutagenic response in (SV-50). The mutagenic activity of nitrosated coal dust could be detected in both (SV-50) and (TA-98) at a dose of 0.29 milligrams of original material per plate (mg/plate). The detection limits for the diesel emission particulate extracts were 0.06mg/plate in (SV-50) and 0.018mg/plate in (TA-98). The authors conclude that the assay system using (SV- 50) is useful for detecting mutagenic activity of complex environmental mixtures.