Previous work revealed substantial levels of mutagenicity in effluents from certain municipal wastewater treatment plants. One of these treatment plants was selected for further study to track the effluent mutagenicity to its sources, to chemically characterize the mutagenicity, and to assess the treatability of the mutagens. Mutagenicity testing using the Salmonella/microsome assay was performed on methylene chloride extracts of influent and effluent samples from the municipal wastewater treatment plant, as well as on four selected industrial effluents entering the plant. The mutagenicity of the influent samples was detected only in the presence of a microsomal metabolic activation system and was highest in Salmonella strain TA98. About two-thirds of the mutagenicity passed through the treatment plant, suggesting that the mutagenic compounds were refractory to conventional biological treatment. No significant mutagenic activity was detected in three of the industrial waste streams, all paper products plant discharges. However, a high level of mutagenicity (1.2 million TA98 revertants/liter) was detected in the effluent from a coke oven plant. The source could account for all of the mutagenicity entering the wastewater treatment plant. After fractionation of the coke oven effluent by sequential extraction at neutral, acidic and basic pH with methylene chloride, 93% of the TA98 (+S9) mutagenicity was found in the neutral fraction. A C18 column fractionation scheme using a methanol/water elution gradient revealed that 92% of the mutagenicity eluted with the 75% and the 80% methanol water fractions.