Twelve municipal wastewater treatment plant sludges were collected and extracted by sequential extraction with methylene chloride and methanol on a Soxhlet apparatus. Each of three sludge fractions, methylene chloride, methanol, and a combined fraction (36 fractions total) was tested in the Salmonella/microsome assay. Only one of the sludge fractions induced a positive response in the absence of metabolic activation, and twelve fractions induced a doubling of revertant colonies at two or more consecutive dose levels with activation. The maximum mutagenic response observed with strain TA98 in a methylene chloride fraction was 120 net revertants at a dose level of 10 mg of extract per plate. The maximum response induced by the methanol and combined fractions was 89 net revertants, induced by a municipal sludge with 20% industrial contribution, and 61 net revertants, induced by a municipal sludge with 19% industrial contribution, respectively, both tested with strain TA98 at a dose level of 5 mg of extract per plate. These results indicate that there will be substantial differences in the mutagenic potential of municipal sludges from a single source and from different sources. The results of a land application study using one of these sludges indicates that some of these mutagenic compounds may be persistent in the soil.