In this study, 345 cattle from 7 herds with a history of lymphosarcoma were tested for antibody to BLV antigens by three serological methods, namely immunodiffusion using a bovine leukemia virus glycoprotein with a molecular weight of 60,000 as antigen, and radioimmunoassay using a bovine leukemia virus glycoprotein with a molecular weight of 60,000 and a bovine leukemia virus protein with a molecular weight of 24,000 as antigen. The three tests under comparison agreed for 335 animals, 240 being negative in the three tests, and 95 being positive. Results were variable in ten cases only. Glycoprotein with a molecular weight of 60,000 antibody titers were systematically higher than were protein with a molecular weight of 24,000 antibody titers in bovine sera and milk, as well as in sera of experimentally infected sheep. In the latter case, antibodies to bovine leukemia virus antigens reached maximal values at the animal death in the tumor phase of the disease. Ratios of serum antiglycoprotein titer to milk titer varied between 4 and 117, showing that, if milk pools are to be used in surveys of bovine leukemia virus infection, use of very sensitive techniques of detection is mandatory.