The mutagenic activity of tobacco snuff was studied by the Ames/Salmonella assay. Two brands of tobacco snuff or coal dust were extracted with dichloromethane, followed by extraction by a 1 to 1 mixture of acetone plus methanol. The snuff and coal dust extracts, alone or in combination, were tested for mutagenicity in Salmonella strains TA-1535, TA-98, or TA-100 with or without S9 mix, obtained from liver homogenates of male CD-rats pretreated with Aroclor-1254. Snuff concentrations ranged up to 85 milligrams per plate. The assays were conducted at pH of 8.2 or 3.0. No mutagenic activity was observed for extracts of either brand of snuff in TA-98 without S9 activation at pH 8.3. Mutagenic activity was found at pH 3.0. Coal dust extracts were non mutagenic at either pH. The mutagenic potency of tobacco snuff was significantly enhanced by coal dust. The mutagenic response of TA-98 at pH 3.0 was greater than that of TA-1535, with or without S9 activation, indicating that the mutations were primarily frameshift mutants. The authors suggest that tobacco snuff inducers may also play a causal role in the elevated risk of gastric cancer in coal miners. Mutagenic substances can form in the human stomach from ingested snuff under acidic conditions that are similar to those in the in-vitro mutagenicity experiment, and these, if carcinogenic, can cause stomach cancer.