The solvent extracts of non-agricultural soil were shown to cause a potent mutagenic response to Salmonella bacteria. The extract of soil from local sites had activities of 100,000-250,000 TA98 revertants per kg with S9 activation and 44,000-83,000 revertants per kg to TA100 with activation. The soil is mutagenic to both strains without S9, but at lower levels. Nitroreductase-deficient bacterial strains show that the soil extract may have mutagenic compound(s) containing a nitro group. The revertant numbers per gram of soil are lowest at the surface, increase at 4-10 cm and decrease in a linear fashion to a depth of 0.5 meter. HPLC fractionation shows that there is more than one separable component responsible for the mutagenicity of the soil. The source and consequences of the high levels of mutagenicity in 'ordinary soil' are unknown. Soil mutagens can be a significant complicating factor to those using the Ames/Salmonella assay for environmental sampling.