Functional activities and surface markers of the neoplastic T cells of three patients with Sézary syndrome were studied and compared with the functional activities of OKT4+ and OKT8+ T-cell subsets from healthy donors. The neoplastic T cells of the patients had the following marker phenotype: OKT1+3+4+6-8-11+17+I1-M1-3A1-. In contrast to healthy donor OKT4+ T cells, none of the neoplastic T cells had significant helper activity on pokeweed-mitogen-driven Ig synthesis. In one patient the neoplastic T cells suppressed pokeweed-mitogen-driven Ig synthesis. The suppression by the patient cells did not require the presence of radiosensitive OKT4+ suppressor-inducer cells in the culture. It is concluded that OKT4+8-3A1- neoplastic T cells from Sézary-syndrome patients lacking helper activity are more common than previously thought and that in some cases the OKT4+8-3A1- Sézary cells can mediate suppressor activity.