Triton X-100 or Nonidet P40-deoxycholate extracts of [3H]fucose-labeled Rous sarcoma virus-transformed chick embryo fibroblasts were examined by indirect immunoprecipitation for the presence of a tumor-specific neoantigen of 100,000 daltons. Extracts were incubated with immune IgG from Rous tumor-sensitized chickens, and the resultant antigen-antibody complexes were precipitated with rabbit antichicken IgG and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Radioactivity appeared between the migration positions of proteins having molecular weights of 65,000 and 95,000 daltons and at about 30,000 daltons. These antigens were group-specific, and thei precipitation could be inhibited by competition with extracts from cultured fibroblasts that had been infected with a nontransforming avian leukosis virus. They were not precipitated with IgG from unimmunized chickens or chickens immunized with the culture supernatants of uninfected chick embryo fibroblasts. In contrast to results reported recently, the present results could not confirm by immunoprecipitation, the existence of a tumor neoantigen different from that associated with viral components. However, the tumor-specific antigen possibly existed on the cell surface but was not preserved in these detergent extracts.