An effective antigen was obtained, presenting a leukosis virus. The culture liquid, and occasionally the cells of the FIK cell line permanently infected with the agent of bovine leukosis served as a source of virus. The antigen, possessing a very good activity was produced by means of precipitation of the virus with 30 per cent ammonium sulfate, dialysis, and additional, concentration with polyethylene glycol-4000 until a 100-fold concentrations as against the initial volume of the virus was attained. Diluted antigen at 1:1 and 1:5 and leukosis serum produced in agar gel a precipitation line for 24 to 48 hours. In terms of activity and specificity this line did not differ from the line formed by glucoprotein antigens obtained from West Germany and the United States. With a normal bovine serum and sera containing Rota, and Parainfluenza-3 antibodies no precipitation line was formed. The good properties of the antigen were confirmed by joint investigations of bovine sera in this country and the Soviet Union with identical results. A total of 26 000 sera were examined with the newly obtained full value antigen, establishing the spread of the leukosis infection in this country as well as the fact that it had been imported from abroad.