Multiparameter studies of an unusual patient exhibiting cutaneous manifestations of both mycosis fungoides and Sezary's syndrome are presented. The neoplastic cells of dermal and nodal infiltrates and peripheral blood expressed both helper and suppressor immunologic phenotypes. Cytofluorographic analysis of cells isolated from lymph node and peripheral blood showed a population of neoplastic cells that were stained with the monoclonal antibodies OKT 3, 4, 8, and 11. Immunoperoxidase staining of frozen sections with monoclonal antibodies Leu 1, 2, and 3 provided a topographical identification of an identically marking population of cells in dermis and lymph node. In light of current models depicting normal T-cell lineage, the authors suggest that the neoplastic population in this patient, expressing both helper and suppressor phenotypes, reflected a phenotypic stage of immunologic maturation (OKT 6-, OKT 10-, OKT 3+, OKT 11+, OKT 4+, OKT 8+) in which the neoplastic cells had not yet segregated into distinctive T-cell subsets. While excess helper activity was suggested by serum hypergammaglobulinemia, in vitro helper and suppressor function was not determined. The range of studies employed illustrates the wide variety of technics required to adequately characterize complex clinico-immunopathologic disorders, as represented by this case, and the wealth of information that can be gleaned.