Further advances in retroviral vaccine development require a better understanding of the antigenic structure of the envelope complex which is directly involved in infectivity events such as receptor recognition and membrane fusion. To design an optimal vaccine against BLV infection, we chose an approach based on the use of synthetic peptides covering 78% of the gp51 sequence in order to select only those segments that could induce a protective response via cellular and humoral immunity. On the other hand, we built a model of the BLV env glycoprotein 3D organization, based upon the very sensitive hydrophobic cluster analysis (HCA). The major information highlighted from this model is that the two loops, against which the most efficient neutralizing antipeptides antibodies are directed against, are in close proximity at the top of the "head" and could represent a potential site for receptor binding. These two peptides are of particular interest since they induce also a helper T-cell response. We further propose that the BLV envelope glycoprotein oligomerizes as a trimer.