Abstract: Monoclonal antibodies, and hybridoma cell lines for their production, that bind with a high degree of specificity proteins associated with HTLV-III virus are presently disclosed. In particular, transmembrane envelope glycoprotein gp41 (41,000 dalton molecular size), major core antigen p24 (24,000 dalton molecular size), and p17 protein (17,000 dalton molecular size) are disclosed. The proteins to which the present monoclonal antibodies respond are essentially antigenically distinct from HTLV-I and HTLV-II. SVM-16 is an IgM monoclonal antibody, SVM-23 is an IgG.sub.2 monoclonal antibody, and SVM-26 is an IgG.sub.1 monoclonal antibody, all of which bind to p24. SVM-25 is an IgG.sub.1 monoclonal antibody binding gp41, and SVM-33 is an IgG.sub.1 monoclonal antibody binding p17. All the monoclonal antibodies of the present invention are produced in hybridoma cells prepared by fusing myeloma cells with spleen cells from mammals, such as mice, immunized with lysates of purified virus.