Primary biliary cirrhosis is characterized by the immune-mediated, nonsuppurative destruction of intrahepatic small bile ducts, with significant T-cell involvement. To date, B cells in livers of primary biliary cirrhosis patients have been ignored in immunopathological evaluations of nonsuppurative cholangitis. This study aimed to correlate the distribution of activated T and B cells with the histopathology of nonsuppurative cholangitis in primary biliary cirrhosis, with emphasis on B cell infiltration. The distribution of activated T and B cells and the histopathology in primary biliary cirrhosis were heterogeneous in bile ducts showing nonsuppurative cholangitis; they were also heterogeneous in various parts of a given bile duct. Although activated T cells were significantly involved in the occurrence of nonsuppurative cholangitis, B cell–predominant reaction or cholangitis and B-cell infiltration into the biliary epithelial layer were also prominent in some nonsuppurative cholangitis. Many bile ducts with nonsuppurative cholangitis of various histological types showed intermixed activated T- and B-cell infiltration. These findings indicate that immune mechanism(s) involving B cells and, probably, humoral immunity are at work in the occurrence of nonsuppurative cholangitis and the destruction of bile ducts in primary biliary cirrhosis. Cell-to-cell interactions of activated T and B cells should therefore be evaluated in livers of patients with primary biliary cirrhosis. (HEPATOLOGY 1993;18:570–575.)