The biological and biochemical properties of the transformation-specific proteins of three avian oncornaviruses with different oncogenic potentials were compared, namely the gag-myc protein of the avian myelocytomatosis virus MC29, the gag-erb A protein of the avian erythroblastosis virus AEV, and the gag-fps protein of Fujinami sarcoma virus FSV. These oncogenes were analyzed in transformed fibroblasts that expressed only the transforming proteins but showed no virus replication. Monoclonal antibodies against the viral structural protein p19, which is the N-terminus of the proteins, were used for indirect immunofluorescence, for immunoprecipitation of the proteins from subcellular fractions, and for immunoaffinity column chromatography. With this last method a 3000-fold purification of the proteins was obtained. By indirect immunofluorescence it was shown that the gag-myc protein was located in the nucleus, and bound to DNA after purification. The gag-erb A protein was not nuclear but probably located in the cytoplasm and did not bind to DNA after purification. Neither of the two proteins exhibited protein kinase activity. In contrast, the gag-fps protein did not bind to DNA but showed protein kinase activity after purification. It was not located in the nucleus either.