Serologic evidence of antibodies in humans to avian leukosis/sarcoma viruses (ALSV) and reticuloendotheliosis viruses (REV) has in general been negative. Because of the difficulty in infecting mammalian cells in vitro with these viruses, it is generally held that they do not infect humans. We first provided presumptive evidence of serologic response to these viruses in human sera of workers in poultry slaughtering plants, using an ELISA. We now provide confirmatory evidence using Western blot assay. Our results show that exposed poultry workers and subjects with no occupational exposure to these viruses have antibodies in their sera specifically directed against ALSV p27, p19, p15, and p12 antigens. In addition, we demonstrate evidence of serologic response to REV. This is the first time definitive evidence of exposure to ALSV or REV has been demonstrated in human sera. The significance of this is not known. Further investigation into whether these findings mean that virus has been integrated into the human genome is needed, to assess the public health implications of these results.