Primary biliary cirrhosis is a disease characterized by slowly progressive intrahepatic cholestasis, destructive lesions of the septal and larger interlobular bile ducts, and granulomas. It is associated with defects of both humoral and cellular immune function. As part of the detailed evaluation of these defects, the status of the complement system has been evaluated. Striking abnormalities of serum complement levels are found but are difficult to interpret. However, the demonstration of marked hypercatabolism of C3, but not albumin, suggests that the complement system may be in a chronically activated state. Furthermore, an unequivocal defect in the clearance of sensitized erythrocytes by receptors for C3b on Kupffer cells has been found. One possible explanation for this finding would be that a large proportion of these receptors are occupied either by immune complexes containing C3b or excess free C3b that is generated by complement activation. Major defects of C3 catabolism and C3b-receptor-mediated clearance are not found in patients with HBsAg-negative chronic active hepatitis. These findings suggest a role for the complement system in the pathophysiology of primary biliary cirrhosis.