This study was designed to identify the site of formation of 1-thiocarbamoyl-2-imidazolidinone (TCI), a potent immunoactive metabolite of the antihelminthic drug, niridazole. When niridazole was administered intragastrically to C57Bl/6J mice, a 4-hr delay was observed before TCI was detected in the serum. By contrast, 4-hydroxyniridazole, a marker of hepatic niridazole metabolism, appeared in the serum within 30 min. Changing the route of niridazole administration from intragastric to intracaecal abolished the lag period in the rise of serum TCI concentrations relative to the 4-hydroxyniridazole marker. Pretreatment of mice with neomycin sulfate reduced the amount of TCI excreted in the urine by about 90% over a 24-hr period, but did not affect the amount of 4-hydroxyniridazole excreted. Injection of niridazole into isolated segments of mouse intestine resulted in TCI production, with the greatest conversion noted in the caecum. Subsequent incubation of niridazole with suspensions of mouse caecum contents in vitro also resulted in the formation of TCI, but not 4-hydroxyniridazole. Attempts to demonstrate TCI formation in vitro with various fractions of mouse liver were unsuccessful. These results indicate a dissociation of TCI formation from the major hepatic pathway of niridazole metabolism and support the view that TCI is formed from niridazole in the gastrointestinal tract as a result of the action of intestinal microflora.