A radioimmunoassay (RIA) for the major internal protein of the bovine leukemia virus (BLV p24) was established using anti-BLV p24 natural antibodies and purified 125I-labelled BLV p24. The final precipitation of the immune complexes was realized by a preparation of inactivated Staphylococcus aureau Cowan I. Sera from 363 cows belonging to (1) leukemic herds, (2) non-leukemic but BLV-exposed herds, and (3) apparently unexposed herds were studied comparatively in BLV p24 RIA, complement fixation and immunodiffusion. The BLV p24 RIA appeared much more senstive than the two other methods in the detection of positive sera. With this method 100% of the leukemic animals, excluding those with juvenile lymphosarcoma, presented very high antibody titers (greater than or equal to 10,000). Practically all cows with persistent lymphocytosis were also positive with slightly lower levels of antibodies, confirming the relationship between BLV infection and the persistent lymphycytosis. Moreover, about two-thirds of the hematologically suspect animals and one-third of the normal animals from BLV-exposed herds were found positive, whereas 100% of the sera from unexposed cows remained negative for anti-BLV p24 antibodies.