The sensitivities of several serological and direct methods for the detection of bovine leukemia virus (BLV) infection in cattle have been critically compared. Among the serological methods, the radioimmunoassay (RIA) using the BLV glycoprotein (gp) was found to be more sensitive than the RIA using the internal virion antigen (p25) and the immunodiffusion test using the virion gp antigen. The differences in sensitivity between these tests were particularly evident with sera of cattle in early stages of infection. The sensitivities of the syncytia induction assay and the competitive RIA for the direct demonstration of BLV infection in cattle are comparable. Following natural infection, antibodies to the gp antigen in most cattle were detected by the RIA several months before infectious BLV became detectable by the SIA in the blood lymphocytes. It is concluded that the most accurate, sensitive, and rapid method for the detection of BLV infection in cattle is the RIA using the virion gp antigen. The data presented confirm that BLV infection as well as the antibody responses of naturally infected cattle to the BLV p25 and gp antigens are persistent.