When boiled pork extract was heated under reflux at 102 degrees C for 4 hr mutagens, which were detected using Salmonella typhimurium strains TA98 and TA1538, were formed. The level of mutagenicity was dependent on the concentration of pork in the extract, the duration of boiling and on pH; the optimum pH for mutagen formation was found to be 9 to 11. Thin-layer chromatographic analysis showed that the mutagens formed in boiled pork extract were chromatographically distinguishable from benzo[a]pyrene and from the primary mutagenic pyrolysis products of tryptophan (3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole and of glutamic acid (2-amino-6-methyldipyrido[1,2-a:3',2'-d]imidazole).