Abstract: Hybridoma cell lines secreting monoclonal antibody (MCA) to avian leukosis virus (ALV) structural proteins p27 and p19 have been established. MCA 6AL20 (IgG1 isotype) reacted with RPL-40, AMV, RAV-2 and Carr-Zilber RSV (CZ-RSV).representing exogenous ALV subgroups A, B and D, respectively, but not the endogenous virus RAV-0 (subgroup E) in an indirect enzyme-linked immunosorbent assay (ELISA). MCA 6AL22 reacted as above and, in addition, reacted with the Prague strain of Rous sarcoma virus (PrC-RSV), subgroup C. Both MCAs im-munoprecipitated p19 from 35S-methionine-labeled RPL-40 or RAV-1,but not RAV-O infected chi-cken embryo fibroblasts (CEF). They can be used to differentiate exogenous from endogenous (RAV-O) infection either in an indirect antibody ELISA or by immunoprecipitation. A third MCA, 6AL42 (IgG2a isotype), reacted with exogenous viruses at an antibody titer up to 1,000-fold higher than with endogenous subgroup E, RAV-O virus in indirect ELISAs. MCA 6AL42 immunoprecipitated p27 and the group-specific antigen precursor protein, pr76, from cells infected with RPL-40, RAV-1 or RAV-O. A double antibody sandwich ELISA, using 6AL42 as the primary binding antibody and conjugated rabbit anti-p27 as the reporter antibody, differentiated between endogenous RAV-O and exogenous p27 antigen in undiluted albumen samples.