2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is a carcinogenic heterocyclic amine formed in meats during cooking. Although the formation of PhIP metabolites by mammalian enzymes has been extensively reported, the involvement of the intestinal bacteria remains unclear. This study examined the urinary and fecal excretion of a newly identified microbial PhIP metabolite 7-hydroxy-5-methyl-3-phenyl-6,7,8,9-tetrahydropyrido[3',2':4,5]imidazo[1,2-a]pyrimidin-5-ium chloride (PhIP-M1) in humans. The subjects were fed 150 g of cooked chicken containing 0.88-4.7 microg PhIP, and urine and feces collections were obtained during 72 h after the meal. PhIP-M1 and its trideuterated derivate were synthesized and a LC/MS/MS method was developed for their quantification. The mutagenic activity of PhIP-M1, as analyzed using the Salmonella strains TA98, TA100 and TA102, yielded no significant response. Of the ingested PhIP dose, volunteers excreted 12-21% as PhIP and 1.2-15% as PhIP-M1 in urine, and 26-42% as PhIP and 0.9-11% as PhIP-M1 in feces. The rate of PhIP-M1 excretion varied among the subjects. Yet, an increase in urinary excretion was observed for successive time increments, whereas for PhIP the majority was excreted in the first 24h. These findings suggest that besides differences in digestion, metabolism and diet, the microbial composition of the gastrointestinal tract also strongly influences individual disposition and carcinogenic risk from PhIP.