The control of the spread of BLV infection among cattle requires very sensitive methods of detection. The BLV-gp51 radioimmunoassay presents great advantages over the other serological methods, including the BLV-p24 radioimmunoassay. This is clear from studies of normal animals from the high-leukemia-incidence region which show that anti-gp51 antibodies reach higher titers than anti-p24 or may even exist alone. Moreover, a sequential survey reveals that the anti-gp antibodies appear earlier. On the other hand, it was not possible to detect a viral antigen expression in the tissues of the infected animals.