When radiolabeled dinitrophenyl-human serum albumin was injected intravenously in mice together with M315 IgA, labeled antigen was specifically transported into bile, but not into saliva, urine, milk or bronchial and intestinal secretions. Ultracentrifugal analysis showed that the antigen transported into bile was intact and partly complexed with IgA. The radioactivity that was present in other secretions regardless of M315 IgA, represented free and degraded fragments of antigen. M315 IgA alone was readily transported into bile, where it was detected at high titer by hemagglutination, but not into other secretions, apart from milk which contained only very low titers. The liver therefore appears to be singularly capable of transporting both free and complexed IgA into its secretion, the bile. © 1982.