Abstract
The major group-specific antigen (gs-a) of avian leukoviruses was detected by radioimmunoassay in extracellular medium of all embryonic cultures prepared from infected chickens. Embryos of birds not carrying an avian leukosis virus infection and of chickens from a leukosis-free flock could thus clearly be distinguished from infected ones. Release of group-specific antigen into the extracellular medium was independent of subgroup or strain of avian leukosis or Rous sarcoma virus. The radioimmunoassay of the group-specific antigen seems to be well suited to complement the COFAL (complement fixation avian leukosis) and RIF (resistance inducing factor) tests in the detection of leukosis virus in infected material.