The mutagenicity of urine samples spike with fried beef extracts was assessed using the Ames Salmonella microsome assay system and compared to the mutagenicity of samples collected from individuals after the consumption of a fried beef meal. All subjects were healthy nonsmokers, and all were maintained on a beef free diet for 24 hours preceding the study. One group of individuals consumed only liquids including coffee, tea, lowfat white milk, fruit and vegetable juices, water, and sugar for the 12 hours preceding the collection of urine samples. A second group was permitted a regular diet. The subjects consumed two 100 gram patties of fried beef after collection of a control urine sample. Urine samples were obtained at varying intervals over the following 24 hours. The tester strains included (TA-98), (TA-100), (TA-1538), and (TA-1535) with and without S9 mix. Urinary mutagenic activity was observable in individuals on the liquid diet within 6 hours after consumption of the fried beef. Microsomal activation was required. Mutagenic activity was observed over 16 hours after beef consumption in individuals on a regular diet. The activity was not correlated with urine volume, creatinine concentration, or specific gravity, and it varied by subject and trial. Extracts of fried beef obtained from Amberlite XAD2 resin columns showed dose dependent mutagenic activity on testing with (TA-98). Extracts obtained from raw and microwave cooked beef samples were not mutagenic. The authors conclude that ingestion of fried beef results in the excretion of mutagens in urine that can interfere with urinary mutagenicity testing.