"A study was conducted to evaluate combined sewer overflows and urban runoff for the presence of chemical mutagens. The Ames Salmonella/microsome mutagenicity test was used as a general biological effects test for the qualitative detection of mutagens in the sanitary environment, including rain, urban runoff, sanitary wastewater, combined sewer overflows, sewage treatment plant effluent, and receiving waters. The Ames test is a relatively sensitive and simple bacterial test for detecting chemical mutagens. Its advantages over long-term animal tests are speed, ease, and relatively low cost. The test employs previously mutated Salmonella typhimurium LT2 bacterial strains that tend to mutate back to their natural state when exposed to mutagenic compounds. Nineteen samples produced a detectable response to one or more of the five S. typhimurium test strains, with or without metabolic activation. Nine of these samples (47%) were of urban runoff in the project area of metropolitan Syracuse (Onondaga County), New York, and they produced 17 of 30 detectable responses (57%). Five of the samples (26%) were from combined sewer overflows, and they produced 7 of 30 detectable responses (23%). The results indicated that urban runoff components that produce a detectable response in the Ames test may be diluted or inactivated in combination with sanitary sewage to form combined sewage, since fewer responses were detected in the latter than in urban runoff."