Previous studies with monoclonal antibodies of the antigenic structure of bovine leukemia virus (BLV) envelope glycoprotein (gp51) have identified three epitopes (F, G, H) directly involved in the infectivity of BLV, F, G, and H lost their reactivity with the respective monoclonal antibodies after treatment with a reducing agent, indicating that these epitopes were conformational. Sequence comparisons between BLV mutants and differential reactivities of urokinase or proteinase K gp51 fragments with monoclonal antibodies indicated that the NH2 moiety of the env protein harbored the three architectural determinants F, G, and H. ELISA tests demonstrated that anti-F, -G, and -H monoclonal antibodies were maximally reactive toward intact virions whereas they showed much poorer affinities for their respective epitopes when presented on a purified protein. Accordingly, an efficient vaccine against BLV infection will include at least the identified gp51 region presented in its native architectural configuration.