The source of activation of T lymphocytes in primary biliary cirrhosis (PBC) is undefined. Hence, T-cell-mediated reactivity against a biliary tract antigenic protein from human bile was studied. The bile protein was fractionated by 30-50% saturated ammonium sulphate and gel-chromatography, and analysed by SDS-PAGE and Western immunoblotting using rabbit antisera to the bile protein. The antisera reacted specifically with human bile duct epithelium. Western blotting of bile proteins showed two major bands, the B1 and B2 antigens. B1 stained for sialoglycoprotein but not lipid, but B2 was negative for both. Cell-mediated reactivity was tested by proliferation of peripheral lymphocytes against B1. Taking the upper limit of the normal range for stimulation indices (S.I.) as less than 1.89 (= mean + 2 SD), a mitogenic response was detected in 14 of 16 patients with PBC (S.I.: 11.7 to 2.3), and in 4 of 15 patients with chronic active hepatitis, but in none of 12 patients with drug-induced intrahepatic cholestasis or obstructive jaundice. The B2 protein was non-stimulatory. Lymphocyte proliferation to B1 in PBC was confined to T cell fractions of peripheral blood leucocytes. There was no cross-antigenicity between B1 and the M2 antigens, according to Western blotting using the rabbit antisera and PBC sera with anti-M2 reactivity. Thus, the B1 biliary protein is a possible source of T cell activation in PBC and hence could be an immunological co-factor in the pathogenesis of this disease.