Retinol binding protein (RBP) in the plasma of rats treated with D-galactosamine was monitored to establish whether its level can be used to evaluate drug-induced hepatotoxicity. Blood was withdrawn by heart puncture at 0 hours and 12 hours after the administration of D-galactosamine (400 mg/kg body weight i.p.) to rats. Lactate dehydrogenase (LDH) and alanine aminotransferase (ALT) in the plasma at 12 hours after the D-galactosamine administration significantly increased, while RBP in the plasma at that time significantly decreased. On the other hand, the albumin in the plasma was unaffected at 12 hours after the D-galactosamine administration. Thus RBP seems to monitor different aspects of drug-induced hepatotoxicity than LDH and ALT and to detect the drug-induced hepatotoxicity more sensitively than albumin level under the present conditions.