Our understanding of the immunobiology of PBC has dramatically changed with the application of molecular biology to clinical medicine. Because of the molecular characterization and identification of the mitochondrial autoantigens, it is now possible to define explicitly mitochondrial autoantigens and examine recognition sites at the primary sequence level. In addition, the expression of cloned antigens has facilitated the development of more reliable assays for mitochondrial autoantibodies. The use of cloned recombinant antigens should, one day, replace the traditional AMA immunofluorescence for diagnostic assays. Possible genetic and environmental factors associated with risk for PBC can also be investigated. It is now also possible to begin the task to defining the role of T cells in the immunopathology of PBC and exploring the issue of whether specific immunotherapy is feasible. There is increasing evidence that PDC-E2 or a similar molecule is located on the cell membrane of biliary epithelial cells. The mechanism for this expression remains to be studied. The explosion of data in PBC is an example of the application of new techniques to investigate old problems. This has occurred because of networking between laboratories in many countries and the generous exchange of sera and donation of livers removed at transplantation. Unfortunately, there is no animal model for PBC; if an animal model was found it would have major importance. Finally, we emphasize the need to study patients early in the course of disease in order to define the events that initiate pathology.