One variety of anti-mitochondrial antibody(AMA) is characteristically found in sera from patients with primary biliary cirrhosis(PBC). The major target antigens of this type of AMA are M2s. It is well known, however, that AMA-negative PBC also exists. An alternative disease concept, called autoimmune cholangiopathy, recently has been advocated. This new concept is defined by the following criteria: 1)the failure to detect AMA and anti-M2, 2)the detection of a diffuse type of anti-nuclear antibody and anti-smooth muscle antibody, 3)pathological findings compatible with PBC, and 4)the effectiveness of prednisolone. However, the difference between AMA-negative PBC and autoimmune cholangiopathy is controversial. Therefore, we analyzed antibodies to four major M2 proteins with Western blotting in 34 cases of immunofluorescent AMA-negative PBC. In 31(91.2%) of these 34 AMA-negative sera, antibodies to at least one of these four major M2 proteins was detected. In serum samples from 34 control patients with AMA-positive PBC, antibodies to at least one of these four proteins were detected in all cases. In addition, we studied the frequency of cases which satisfied the serological criteria of autoimmune cholangiopathy. In only one(0.7%) of 141 cases was the serological criteria met. We conclude that to clarify the serological differences between autoimmune cholangiopathy and AMA-negative PBC, the analysis of M2 proteins by Western blotting is essential.