The mutagenic potential of chewing tobacco with or without nitrites was studied using the Salmonella microsome assay. Chewing tobacco extracts (CTE) were prepared from two commercially available brands of chewing tobacco. A polar CTE was prepared by sequential extraction with dichloromethane and a mix of equal parts acetone and methanol. This material was filtered, concentrated, and resuspended in 50 percent dimethyl-sulfoxide. A nonpolar CTE was prepared by extraction with water. This mixture was centrifuged and the supernatant was concentrated by freeze drying. A portion of each extract was further treated with sodium-nitrite (7632000). Mutagenicity of these extracts was tested using the standard Ames Salmonella assay system. Mutagenic activity was observed only in extracts treated with nitrite. This activity was not present when ascorbate was added to the CTE during the reaction with sodium- nitrite. According to the results of this study, chewing tobacco alone was not mutagenic. It is stated that nitrites have been previously documented as potent nitrosating agents, and nitrites of dietary origin have been demonstrated in saliva. The authors suggest that an in-vivo nitrosation of chewing tobacco is possible, and may account for the adverse effects of chewing tobacco on the oral cavity, pharynx, and larynx.