Selected populations of cats that were naturally exposed to the feline leukemia virus (FeLV) were found to have humoral antibodies to a normal cell protein designated NCP105. Earlier studies revealed that cats exposed to FeLV often had serum antibodies to the feline oncornavirus-associated cell membrane (FOCMA) as well as to a feline sarcoma virus (FeSV) -specific transforming protein designated gag-fes. Cats with no history of exposure to FeLV or FeSV lacked antibodies to all three antigens: NCP105, FOCMA, and gag-fes. Following exposure to FeLV, cats develop antibodies to either NCP105 or to gag-fes and FOCMA, but not to both groups of antigens. NCP105 is present in both normal and transformed cells from a wide variety of species. It lacks peptide homology with gag-fes and it is not a phosphoprotein. The presence of antibodies to NCP105 in cats exposed to FeLV but not in unexposed cats suggests that FeLV may activate the NCP105 gene or increase the relative immunogenicity of this protein in vivo.