1. Incubation of human or rabbit faeces with sulphinpyrazone gave greater reduction under anaerobic than under aerobic conditions. Reduction of sulindac by human faeces was more extensive than that of sulphinpyrazone. 2. Growth of mixed cultures of intestinal bacteria in nutrient media containing antibiotics produced a marked inhibition in their ability to reduce sulphinpyrazone. Sulphide formation was inhibited by metronidazole and lincomycin for human faeces and by tetracycline for rabbit faeces/caecal contents. 3. The formation of the sulphides of sulindac and sulphinpyrazone ex vivo was decreased in faeces from patients treated with metronidazole. Metronidazole, but not tetracycline, decreased the extent of reduction of sulphinpyrazone by rabbits in vivo. No reduction of either substrate occurred on incubation with ileostomy effluent. These data indicate that anaerobic intestinal bacteria are important in the reduction of these sulphoxide-containing drugs. 4. However, when incubated anaerobically with over 200 strains of bacteria isolated from human faeces, sulphinpyrazone was reduced by most of the aerobic but not the anaerobic organisms. Sulindac was reduced more extensively by the same aerobes and by some anaerobes. 5. The discrepancy between the apparent importance of anaerobes in vivo and in vitro may be due to their very large number present in the hind gut and to the production of an anaerobic environment suitable for the enzymic activity of other organisms, such as aerobes or facultative anaerobes.