Recent work has shown that there is antigenic similarity between common bacterial proteins and epitopes on biliary epithelial cells. Following evidence that the sera of all of a series of patients with primary biliary cirrhosis contained antibodies to an extract of Mycobacterium gordonae (a ubiquitous environmental organism) and that these antibodies cross-reacted with the major mitochondrial M2 epitopes, the study below has been performed to investigate this phenomenon further. Using Western blotting of membrane extracts of Mycobacterium gordonae and seven other atypical mycobacterial species, no such antibodies were detected in the serum of patients with primary biliary cirrhosis, but antibodies to a 65 kDa mycobacterial protein were found in most patients with primary biliary cirrhosis as well as in normal controls and patients with other chronic liver diseases.