GENETIC analyses of avian sarcoma viruses (ASV) have led to the identification of a gene, designated src, which encodes a product required for the initiation and maintenance of neoplastic transformation in infected fibroblasts^1–5. Because the src gene product has not been identified biochemically, this study was initiated to detect a transformation-specific protein, using serum from rabbits bearing ASV-induced tumours. We describe here the identification of a 60,000-MW transformation-specific antigen detectable in ASV-transformed chicken cells and ASV-induced hamster tumour cells by immunoprecipitation of radiolabelled cell extracts with serum from tumour-bearing rabbits. Moreover, the expression of this antigen is temperature dependent in chicken cells transformed by an ASV temperature-sensitive mutant in the src gene. The use of this antiserum may lead to the unequivocal identification and characterisation of the ASV src gene product and this, in turn, may lead to the elucidation of the mechanism of ASV-induced oncogenesis.