Four monoclonal antibodies, human T cell leukemia-lymphoma virus (HTLV) 6, 7, 8, and 9, which react with the 24,000 dalton internal core protein of HTLVI, have been developed. These monoclonal antibodies reacted with only HTLV-infected cells and not with a broad spectrum of normal, neoplastic, mitogen-stimulated, or virus-infected cells and tissues. HTLV 6, 7, 8, and 9 identified at least two different antigenic determinants on HTLV p24 that were also recognized by antibodies present in HTLV+ patient sera. Monoclonal antibodies HTLV 6, 7, 8, and 9 reacted in indirect immunofluorescence assays with HTLV p24 localized at the cell surface of 5-d cultures of HTLV-infected T cells and, as well, reacted with T cells infected with HTLVII, a new type of HTLV isolated from a patient (MO) with a T cell variant of hairy cell leukemia. Thus, HTLV 6, 7, 8, and 9 should prove to be useful diagnostic reagents in the identification of HTLV- and HTLVII-infected T cells.