The metabolic degradation of chloramphenicol has been investigated in bacteria, the rat, guinea pig, dog and man. Following the administration of chloramphenicol to man, the antibiotic is rapidly absorbed and excreted in the urine. A small amount of the antibiotic is excreted unchanged (Ley et al., 1948), but the principal excretory product in man is the conjugate with glucuronic acid, which has no microbiological activity (Glazko et al., 1950). A similar excretion pattern was found in lower animals following oral as well as intravenous administration, but the recovery of total nitro compounds in the urine was considerably less than that found in man (Glazko et al., 1949b). In the case of the rat and guinea pig, a large percentage of the administered dose, following initial absorption, is excreted as the glucuronide via the bile into the intestinal tract. The intestinal flora hydrolyzes the glucuronide and reduces the chloramphenicol moiety to aryl amines which may be partly reabsorbed