Introduction: Primary biliary cirrhosis is an autoimmune liver disease in which cellular and humoral immune responses towards cholangiocytes cause progressive intrahepatic bile duct loss. Cholestasis, liver fibrosis, and eventually cirrhosis result, with liver transplantation or death occurring in end stage disease. The etiology of the disease is unknown, but is thought to involve a permissive genetic predisposition and an environmental insult. Plausible environmental insults include xenobiotic exposure, and bacterial or viral infections. Previous work in our group suggests infection by human betaretrovirus, a virus with close similarity to mouse mammary tumor virus, may be associated with PBC. We are developing an ELISA based assay with the goal of establishing whether patients with primary biliary cirrhosis produce antibodies to human betaretrovirus. We also seek to produce a diagnostic indirect ELISA for detecting prevalence of human betaretrovirus in patients with primary biliary cirrhosis and the general population. Methods: Human betaretrovirus Surface (gp52 Su) protein was expressed as a recombinant antigen from both E. coli and human cells. Affinity tags co-expressed with the antigens, GST for E.coli antigens and polyhistidine residues for mammalian antigens, were utilized to purify expressed antigen. Western blot was used to confirm successful antigen expression. ELISAs were developed and optimized for both E. coli and mammalian expressed antigens. These ELISAs were utilized to probe healthy control and PBC patient serum samples for antibodies against human betaretrovirus surface protein. Western blot was used to validate the findings from the mammalian antigen ELISA.