Previous studies in our laboratory on the co-carcinogenic effect of the ingestion of an industrial carbon black (CB) on chemically induced colon cancer in rats and mice demonstrated no differences in tumor incidences attributable to CB feeding. The present study examined the effect of CB ingestion within the context of a high fat diet, formulated to simulate the typical diet of western industrialized nations. Corn oil was added to ground commercial chow at 20% by weight and CB added at 2.05 g/kg diet and fed for 52 weeks to female Sprague-Dawley rats. Colon tumors were induced with 16 weekly injections of 1,2-dimethylhydrazine (DMH, 10 mg/kg body weight). Tumor incidences in DMH-treated rats ingesting CB were significantly (P less than 0.05) higher than in those with no CB added to the diet (76% vs. 60%). The survival of CB + DMH treated animals (64%) was also lower than that of animals treated with DMH and not ingesting CB (80%). These findings may implicate CB ingestion as a co-carcinogen for industrial workers when acting in synergism with high fat diets and other unknown colon carcinogens.