Colonic biopsy specimens from patients with ulcerative colitis and normal subjects were studied for the ability to metabolize an environmental carcinogen, benzo(a)pyrene. Approximately 73% of 30 colonic biopsy specimens from 7 ulcerative colitis patients could metabolize benzo(a)pyrene to oxidized products, with an average production of 11.6 nmol/mg biopsy protein. In contrast, 39% of 23 biopsy specimens from 5 normal persons showed metabolic activity, with an average of 2.79 nmol benzo(a)pyrene metabolites/mg biopsy protein. Thus, benzo(a)pyrene oxidation activity in colonic tissue from colitis patients was, on the average, fourfold greater than that in normal subjects. This elevated metabolic activity appeared to be unrelated to the state of inflammation in the biopsy section. There was a tendency toward increased metabolic activity in the distal colon. Although there is no evidence that benzo(a)pyrene itself is "the colon carcinogen," this chemical belongs to a broad class of environmental carcinogens, the polycyclic aromatic hydrocarbons. Our findings suggest that the colonic mucosa of patients with ulcerative colitis has a greater ability than that of normal subjects to oxidize such chemicals possibly to electrophiles with higher mutagenic potential.