Chick embryo fibroblast cells (CEF) infected with avian leukosis viruses were stained selectively by the indirect immunoperoxidase method. Good results were obtained by the use of a successive combination of periodate-lysine-paraformaldehyde fixation and diaminobenzidine reaction mixture. Viral antigens were detected type-specifically on infected cells. Type-specific antisera determined by the neutralization test were absorbed by the homologous type of virus-infected CEF, but not by the heterologous type of these cells. This test was more effective for detecting virus infectivity than the resistance-inducing factor test. Viral antigen was observed 2 days after inoculation with a large amount of the virus. The minimum infective dose of the virus for the antigen detection was 100 resistance-inducing units (RIU) per plate 4 days after infection, or 1 RIU per plate in CEF after two passages.