It has been proposed that polymeric IgA is translocated from plasma to bile across hepatocytes of the rat liver by a secretory component-mediated, vesicular transport. To define the ultrastructural details of the proposed transport mechanism, we employed peroxidase-labeled antibody immunocytochemistry to localize secretory component in the rat liver and monitor the hepatic translocation of homologous myeloma polymeric IgA infused i.v. Secretory component was found associated with the endoplasmic reticulum, Golgi complexes, cytoplasmic vesicles, and plasma membranes of the sinusoidal and canalicular surfaces of hepatocytes; secretory component at the sinusoidal surface was most prominent in micropinocytic invaginations or pits. Livers were examined for the sites of polymeric IgA 5, 15, and 30 min after infusion. Evidence was obtained that polymeric IgA is translocated across hepatocytes by a series of events: 1) polymeric IgA binds selectively to secretory component on the external surface of the sinusoidal plasma membrane; 2) secretory component-IgA complexes are internalized in endocytic vesicles; 3) the vesicles migrate through the cytoplasm without association with lysosomes or Golgi complexes; 4) the vesicles fuse with the cytoplasmic surface of the bile canalicular membrane, where secretory component-IgA complexes are released into bile by exocytosis.