Liver specific protein (LSP) is known to be a macrolipoprotein of complete organ-specificity but without complete species-specificity. This membrane antigen is believed to play an important role in the pathogenesis of human and experimental chronic active hepatitis (CAH). In the present study, we investigated the species-crossreactivity of LSP by crossed immunoelectrophoresis, tandem crossed immunoelectrophoresis and fused rocket immunoelectrophoresis. With a sheep anti human LSP serum, two determinants of human LSP could be detected--one was found to be species-specific, the other crossreacted with rabbit, rat, swine and mouse LSP; no reaction was found with bovine and sheep LSP. A rabbit anti human LSP serum, after short term immunization, reacted only with a species-specific determinant of human LSP, no species-crossreactivity was observed. In contrast, rabbits with experimentally induced CAH, after longterm immunization with human LSP, had developed an autoantibody to rabbit LSP in addition to antibodies to the species-specific determinant of human LSP. Antibodies to the liver membrane antigen (LM-Ag) could not be detected. In conclusion human LSP contains a species-specific and a non-species-specific determinant. CAH in rabbits is induced by the loss of tolerance to the non-species-specific determinant.