Résumé : Fifteen monoclonal anti-BLV gp51 antibodies are characterized. Competition antibody binding assays show that they are directed against eight independent antigenic regions on the BLV gp51 molecule. Conformation or accessibility of some of these gp51 epitopes change with the test system used, namely the liquid phase radioimmunoassay with radiolabeled antigen or the solid phase enzyme immunoassay with plastic bound gp51 or BLV particles. A two-site immunometric assay using monoclonal antibodies directed against two independent epitopes allows detection of isolated gp51 molecules at a minimal concentration of 0.4 ng/ml and is also suitable for the detection of BLV particles. © 1982.