Compounds mutagenic toward Salmonella typhimurium strains sensitive to frameshift mutation (1537, 1538 and TA98) were formed when fish flesh was fried at 190 degrees c. Four species of marine fish commonly consumed in the United States were cooked in an electric skillet and broiled beneath the elements of an electric oven. Organic extracts of the fish were tested in the Salmonella mutagenic assay using strains 1535, 1537, 1538, TA98 and TA100. Basic organic extracts of fried but not raw or broiled samples exhibited significant mutagenicity with metabolic activation. Mutagenic activity ratios ranging from 3.3 to 15.7 for the extract from 20 g of fish were observed. The mutagenicity produced during the frying of fish was dependent on time. Frying times of less than 6 min produced no mutagenic activity, while at 6 min or greater substantial mutagenicity was generated.