ABSTRACT— Monoclonal antibodies (mc/anti-LSP) have been prepared by polyethylene glycol fusion of P3/NSI/Ag4–1 myeloma cells with spleen cells from Balb/c mice hyperimmunized with human liver-specific membrane lipoprotein (LSP). Ten hybridomas, cloned by limiting dilution, produced mc/anti-LSP reacting (by ELISA) with human LSP but not with normal human plasma proteins nor with a variety of other proteins likely to co-purify with LSP. Three of these (A15/7, A9/63 and B20), producing high-titre IgG1 mc/anti-LSP, were biosynthetically radiolabelled and used as index antibodies. By competitive inhibition of binding of the index antibodies to LSP in an immunoradiometric assay, the ten hybridoma products were classified into four distinct groups according to their specificities for different epitopes in LSP. None of the index antibodies reacted, on ELISA, with glutaraldehyde-fixed PLC/PRF/5, Chang, Daudi or HSB-2 cell lines nor with human peripheral blood leucocytes. However, A15/27 (but not A9/63 or B20) reacted with saponin-permeabilized PLC/PRF/5 and Chang cells and also with rabbit LSP. The results emphasize the polyantigenic nature of LSP and indicate that at least one of the mc/anti-LSP (A15/27) recognises a species crossreactive antigen that is present in liver-derived cell lines.