Two murine monoclonal antibodies (BE1 and BE2), produced by using leukemic helper T cells from a patient with cutaneous T-cell lymphoma (CTCL) as immunogens, reacted selectively with CTCL lymphocytes and some transformed cultured lymphocytes, as determined by radioimmunoassay (RIA) and indirect immunofluorescence (IIF). BE1 reacted significantly (P ≤ 0.001) with leukemic CTCL lymphocytes and with CTCL cells from infiltrated lymph nodes (RIA, mean±SD = 776±275 cpm), as compared with background counts (263±68). BE1 binding to normal blood mononuclear cells (RIA, mean±SD = 283±58 cpm) was indistinguishable from background. BE1 also reacted with Epstein-Barr virus (EBV)-transformed B-cell lines (RIA, mean±SD = 794±230) and some long-term T-cell lines. BE1 did not react with the majority of lymphoid cell lines or tumor cell lines tested. BE1 also did not react with any normal tissues screened by IIF. BE1 precipitated a molecule from CTCL cells that, under reducing conditions, has two components with molecular mass of 27,200 and 25,800 D. BE2 also reacted significantly (P ≤ 0.001) with CTCL cells from 2 of 4 patients (RIA, mean±SD = 519±113 cpm). The binding of BE2 to normal mononuclear cells was indistinguishable from background (309±38 cpm). BE2 also reacted with an antigen present on EBV-B-cell lines (RIA, mean±SD = 654±194) and MOLT 3 and HUT 78 T-cell lines. BE2 reacted with an antigen expressed on a subpopulation of lymphocytes from 5 of 8 patients with B-cell CLL studied by IIF (mean±SD = 18±6). Other long-term T-cell lines and tumor cell lines studied by IIF were unreactive with BE2. BE2 did not react with any of the normal tissues studied. BE2 precipitated a molecule (78,000 D) from CTCL cells and EBV-B cells with a single component under reducing conditions. Immunoperoxidase-labeled BE1 and BE2 reacted with CTCL cells in frozen sections of infiltrated lymph nodes and skin. In addition, BE1 and BE2 reacted with blood lymphocytes from 16 of 21 patients whose CTCL had otherwise been considered localized to skin. These two monoclonal antibodies react with tumor antigens associated with CTCL and appear to be useful in the diagnosis of this disorder.