Hepatic bile IgA from 6 patients was measured by radioimmunoassay and characterized immunochemically. The concentration of IgA was 0.14 to 0.88 mg/ml. IgA associated with secretory component (SC) as well as unassociated with SC was demonstrated in all 6 samples; The proportion associated with SC (secretory IgA) was 72% to 95%. IgA and SC were localized immunocytochemically in liver and bile duct tissues by the peroxidase-labeled antibody method. Evidence favoring endocytic, SC-mediated transfer of IgA by intrahepatic and extrahepatic biliary epithelium, but not by hepatocytes, was found.