The molecular and cellular parameters of the transport of IgA immune complexes (IgA-IC) from the circulation to the bile in vivo were studied in mice. Polymeric MOPC 315 IgA was effective in the transport of 125I-DNP-HSA into the bile, whereas complexes containing monomeric IgA were not transported. The transport of IgA-IC could be blocked by excess free IgA of different antigen specificity, but not by IgG or IgM. Hepatobiliary transport did not appear to involve the galactose- or mannose-specific glycoprotein receptors of the liver. No difference in IgA-IC transport, relative to controls, could be seen in mice that had been treated with cobra venom factor or whose mononuclear phagocytic system had been blockaded with colloidal carbon. These data suggest that the hepatobiliary transport of IgA-IC proceeds by a mechanism similar to the selective transport of free polymeric IgA through the hepatocyte.