The acid-mediated mutagenicity of extracts of two leading brands of American tobacco snuff was studied, and attempts were made to elucidate the possible mechanism of this mutagenicity. The snuff extracts prepared included a nonpolar extract prepared by use of dichloromethane (DCM); a polar extract prepared from the DCM extract by use of a mixture of methanol and acetone; and a water extract. Extracts were adjusted to pH 3.0 with hydrochloric-acid and were assayed for mutagenicity, nitrite content, and nitroso compound formation. Extracts mixed with ascorbate solutions were tested for mutagenic activity and nitroso compound formation. In addition, the snuff itself was tested for bacterial contamination. Only acid treated polar solvent extracts showed mutagenic activity. High amounts of nitrite were found in polar solvent and water extracts, but only trace amounts in nonpolar extracts. The amounts of nitroso compounds in the acid treated extracts corresponded to their mutagenic activities, but the presence of ascorbate inhibited these activities. Bacteria from snuff were able to reduce nitrate to nitrite, and the amount of nitrite in snuff extracts was further increased by incubation of these extracts with the bacteria. The authors suggest that the acid mediated mutagenicity of snuff extracts is, at least in part, due to the nitrosation reaction between certain constituents of snuff and nitrite at the low pH.