The peripheral blood lymphoid cells of five patients with Sézary syndrome (SS) were examined with respect to their surface membrane characteristics and their response to mitogens. These cells showed markedly defective mitogenic responses to a broad dose range of phytohaemagglutinin (PHA), pokeweed mitogen, concanavalin A, and a rabbit antihuman lymphocyte antiserum (ATS), when compared with normal human lymphocytes. SS lymphoid cells (three patients studied) also displayed diminished or nearly absent capacity to form rosettes with unsensitized sheep erythrocytes (E-rosettes), and lacked surface immunoglobulin determinants. Despite their poor mitogenic response to ATS, they were as susceptible as normal lymphocytes to ATS-induced, complement mediated cytotoxicity. By comparison with lymphocytes from patients with chronic lymphocytic leukaemia, however, SS lymphoid cells showed decreased susceptibility to leukoagglutination by PHA. By way of contrast, three patients with mycosis fungoides having normal-appearing peripheral blood lymphocytes showed normal lymphocyte responses to mitogens, as well as normal proportions of E-rosette forming and surface immunoglobulin-bearing lymphocytes. These studies demonstrate that the SS lymphoid cell may, in some cases, lack surface properties and mitogen response characteristics of both B- and T-lymphocytes.