Using antibodies directed to beta 2-microglobulin (b2-m) and HLADR antigens, the expression of MHC products by normal and abnormal bile ducts in 90 paraffin-embedded biopsies showing various liver diseases, was studied. Normal and abnormal bile ducts constantly expressed b2-m. Increased b2-m expression was found in 17/19 PBC, and 4/7 chronic aggressive hepatitis or cirrhosis of viral etiology with hepatitic bile duct lesions. Normal bile ducts failed to express HLADR antigens. Aberrant HLADR display was found in 24/26 PBC and 10/16 chronic aggressive hepatitis or cirrhosis of viral etiology with hepatitic bile duct lesions. It is concluded that the pattern of the major histocompatibility complex (MHC) display does not discriminate between PBC and hepatitic bile duct lesions. Enhanced expression of class I MHC products at the surface of medium-sized bile ducts in PBC may render these structures more susceptible to lysis by cytotoxic T-cells, whereas its significance in chronic aggressive hepatitis or cirrhosis remains unknown. Aberrant expression of HLADR antigens by abnormal bile ducts in PBC and chronic aggressive hepatitis or cirrhosis of viral etiology is probably induced by gamma-interferon, liberated by intra-epithelial lymphocytes, and may serve to enhance the immune response, either by attracting HLADR-restricted cytotoxic T-cells or by the presentation of non-self antigens at the surface of bile duct epithelium.