Following the administration of thiamphenicol (TP), the metabolites found in the urine and bile of rat, guinea pig and rabbit, and in the urine of human were unchanged TP, a hydrolysis product of TP, and a conjugate of TP with glucuronic acid. TP glucuronide was isolated from the guinea pig urine and bile, and identified. The urinary and/or biliary excretion of TP metabolites in rat, guinea pig, rabbit and human was also studied by comparison with chloramphenicol (CP). The major route of the excretion of TP in human, rabbit and rat was by way of the kidneys, and the most of the drug was excreted in the unchanged form. In guinea pig, TP was excreted mainly into the bile and the most of the drug excreted was a glucuronide. On the other hand, CP was extensively conjugated and metabolized in all the animals used. In human, rabbit and guinea pig the major route of the excretion of CP was by way of the kidneys, and in rat by way of the bile and the excreted form was mostly a glucuronide.