Monoclonal antibodies were derived from hybrid cell lines produced by fusing mouse myeloma cells with spleen cells from mice recovering from Friend virus-induced erythroleukemia. Of the 17 clones characterized, two appeared to have the Friend, Moloney, Rauscher pattern of specificity. One of these was specific for the envelope protein, gp70, and the other reacted with a core protein, p15. Seven other anti-gp70 clones and one anti-p15 clone were restricted in reactivity to cells infected with Friend or Rauscher viruses only. One clone reacted with p15E and recognized this protein on many ecotropic and dual-tropic viruses. In addition, six IgM antibodies were obtained which appeared to recognize nonviral antigens present only on leukemia cells of the erythroid lineage. Six monoclonal antibodies of complement-fixing immunoglobulin classes with specificity for gp70 or p15 were compared for their ability to bind or lyse erythroleukemia cells in the presence of complement. Individual antibodies to the same viral protein appeared to differ markedly in their ability to mediate cytolysis. © 1981.