Suitable quantities of antiserum for routine COFAL testing may be readily obtained at relatively low cost by the inoculation of newborn hamsters with a chorioallantoic membrane suspension of Schmidt-Ruppin strain of avian sarcoma virus. Serum from hamsters which do not develop easily observable cutaneous tumours within 3 months may contain as much group specific complement-fixing antibody as serum from animals which do develop such tumours.