Primary biliary cirrhosis (PBC) and autoimmune hepatitis (AIH) present with distinct clinical features. The term "PBC-AIH overlap syndrome (OS)" has been adopted to describe the condition characterized by occurrence of both PBC and AIH, although this clinical entity is difficult to define. This study aimed to assess the utility of IgG, IgM, and CD138 immunohistochemistry in the evaluation of AIH, PBC, and OS. Immunohistochemistry was performed with anti-human IgG, IgM, and CD138 to detect specific plasma cells in the liver. Predominant IgG staining was observed in AIH (85.7 %), while equivocal (46.1 %) or predominant (38.5 %) IgM staining was observed in PBC. In OS, equivocal (20 %) or predominant (80 %) IgG staining was observed. The IgM/IgG ratio was significantly higher in PBC than in AIH or OS (P < 0.005). Histological findings revealed significantly higher IgM expression in PBC at cholangitis activity grades 2-3 compared to those at cholangitis activity grades 0-1. In contrast, a significantly higher IgG expression was observed in PBC at hepatitis activity and fibrosis grades 2-3 compared to those at hepatitis activity and fibrosis grades 0-1. Taken together, periportal plasmacytic infiltrates with variable immunohistochemistry patterns of IgG and IgM expression characterized different autoimmune liver diseases.