Summary Sera from hamsters bearing tumours induced by the Schmidt-Ruppin strain of Rous sarcoma virus (sr-RSV) contain antibodies to the avian leukosis group specific antigen and an antigen of chick origin. The ‘chick embryo antigen’ is found in chick embryos infected and uninfected with avian leukosis viruses and in chicken tumours induced by avian leukosis viruses; for example, Rous sarcoma wing web tumour and plasma from avian myeloblastosis virus infected chickens. The ‘chick embryo antigen’ has been detected in sr-RSV hamster tumour cells. Cultured chick fibroblasts do not contain the ‘chick embryo antigen’ and thus the avian-leukosis group specific antigen can be specifically detected by complement fixation using sr-RSV tumour-bearing hamster sera. This COFAL test is as sensitive as the Rubin interference test (RIF test) and unlike the RIF test will detect all known avian leukosis viruses.