Human tissues from adult, newborn/infant, and fetal livers, and gall bladders were immunostained with rabbit antibodies against fatty-acid-binding protein (FABP) isolated from rat liver. These antibodies cross-reacted with FABP from human liver by a double immunodiffusion method. The antigenicity of FABP of human liver is well preserved in various fixatives including 10 per cent formaldehyde, 4 per cent paraformaldehyde, and Zamboni's and Bouin's solution. FABP immunoreactivities, moreover, could be detected similarly in both cryostat and paraffin sections. FABP of hepatic type was found in hepatocytes most frequently in the fetus (as early as the 7th week of gestation), moderately in the newborn/infant, and infrequently in adult liver tissues. The distribution pattern of the positive hepatocytes was uniform throughout the acini in fetuses, but periportal in adults. No immunoreactive cells were found in the gall bladder or in the extra- and intra-hepatic bile ducts. In this study, FABP was found in human hepatocytes in both fetal and postnatal livers. FABP, therefore, might be a useful marker substance in the investigation of the physiology and pathology of the human liver.