14C-Carmoisine, a sulphonated azodye, 200 mg/kg b.w. (25 muCi), was administered to rats by gavage. Separation of radioactive compounds in faeces and urine of these animals was carried out by HPLC with a UV and a radioactivity detector. In addition to unmodified carmoisine, five radioactive compounds were present. The main peak showed both the retention time and the UV spectrum of naphtionic acid. Metabolic patterns similar to those observed "in vivo" were found by incubation of 14C-carmoisine under anaerobic conditions with a bacterial suspension isolated from human faeces and from the intestinal contents of rats. This procedure permits the preparation of amounts of unknown metabolites suitable for their identification.