The effect of supplemental ascorbic acid and alpha-tocopherol on fecal mutagenicity was examined in 2 studies involving 20 healthy human donors aged 22-55 years. The vitamins were given at a dose of 400 mg daily each. The mutagen was extracted from individual frozen feces samples with dichloromethane, and assayed with Salmonella Typhimurium tester strain TA100 without microsomal activation. In the first study, with a single donor on a controlled diet, the fecal mutagenicity decreased (P less than 0.001) on treatment to 21% of control. In the second study, with 19 donors on free-choice diets, the mutagenicity in producers on treatment decreased (P less than 0.01) to 26% of control. Addition of ascorbic acid and alpha-tocopherol directly to feces led to no change in mutagenicity. Antioxidants in the diet may have a role in lowering the body's exposure to endogenously produced mutagens.