Biological transformation and detoxification of 7,12-dimethylbenz(a)anathracene (DMBA) were studied in a nonacclimated sandy loam soil. Parent 14C DMBA biodegraded extensively (62% to 20%), accompanying an increase of metabolite 14C fraction (4% to 53%). Incorporation of DMBA into non extractable soil residue ,4C increased from 12 to 17%, but the increase was not statistically significant. DMBA was transformed into several metabolic products in the soil system, including 4-hydroxy-, 5-hydroxy-, and 10-hydroxy-DMBA and 7,12-dihydro 12-methyl-7-methylene-benz(a)anthracene-12-ol. High polarity transformation products of DMBA demonstrated a negative mu tagenic response with the Ames mutagenicity assay, strain TA 100, for both low and neutral pH soils. Moderate and low polar metabolites, however, induced mutagenicity for both soil samples. The mutagenicity of these metabolites decreased with incubation time in the soil, suggesting detoxification and assimilation of this polyaromatic hydrocarbon in soil systems. Mutagenic responses for the metabolites formed from low and neutral pH soil were similar. J. Water Pollut. Control Fed. 60, 1822 (1988).