Monomeric (7S) IgM was detected by polyacrylamide/agarose gell immunodiffusion and Sephadex G200 gel filtration in 33% of sera from patients with primary biliary cirrhosis (PBC) and 5% of patients with HBsAg-negative chronic active liver disease (CALD). It was not found in the sera of patients with extrahepatic cholestasis, alcoholic liver disease (ALD), HBsAg-positive CALD and normal control subjects. In the PBC group the presence of 7S IgM was associated with significantly higher IgM concentrations and Clq binding activity (P less than 0.001; P less than 0.01 respectively). The antibody specificity of the 7S IgM is unknown. Its presence in patients with high serum IgM concentrations probably reflects failure of complete polymerisation of 7S IgM because of an increased rate of synthesis of the protein. The association of the presence of 7S IgM and high total IgM with immune complexes suggests that the increased rate of synthesis of IgM and formation of immune complexes probably are a result of the same antigenic (or mitogenic) stimulus.