Mutagenic activity has been demonstrated in urine of human subjects after ingestion of fried pork or bacon. Activity was detected with Salmonella strains TA1538 and TA98 particularly, in the presence of liver homogenate S9, and was not enhanced by prior incubation of urine with beta-glucuronidase. Chemical and biological characteristics of the urine activity closely resemble those found in extracts of fried pork and bacon which also increase the frequency of sex-linked recessive mutations in Drosophila melanogaster. Microwave-cooked meat neither contained extractable mutagenic activity, nor contributed to urinary mutagenicity, possible due to the paucity of browning reactions in meat cooked under these conditions. If the urine and meat factors are chemically identical, then approximately one-third of the food activity is recovered from the urine. These results show that mutagenic factors, generated during cooking of pork and bacon, are ingested and absorbed and are subject to urinary clearance in biologically detectable quantities. It is possible that the potential for genetic toxicity in humans of these and related factors has been underestimated.