Anti-T-cell monoclonal antibodies (LEU series) immunoperoxidase technique study for the presence of T cells in cutaneous lesions from four patients with Sézary syndrome and one patient with chronic T-cell leukemia showed that most dermal-lymphoid cells from three patients with Sézary syndrome were reactive with monoclonal antibodies to anti-pan T-cell (LEU-1) and helper T-cell (LEU-3a) subsets but not with those to suppressor-cytotoxic T-cell (LEU-2a) subsets. One patient with progressive disease had atypical dermal-lymphoid cells positive for pan T-cell (LEU-1). Epidermotropic cells were reactive to LEU-1 in all four patients, LEU-2a in one patient, and LEU-3a in one patient. Neoplastic cells in skin lesions of chronic T-cell leukemia showed strong positive staining with LEU-1, but were reactive with both anti-T-cell subset, monoclonal antibodies. The atypical, dermal-lymphoid cells in Sézary syndrome represent mature, helper T cells in most cases. The absence of T-cell subset antigens in one patient with fulminant Sézary syndrome and the finding of both T-cell subset antigens on T-cell leukemia cells suggest the presence of actively proliferating, immature T cells in those cases.