The neoplastic T cells from five patients with adult T cell lymphoma/leukemia (ATLL), born in the Caribbean, were studied with respect to immunoregulatory activity on pokeweed mitogen (PWM) driven immunoglobulin (Ig) synthesis as well as surface-marker phenotypes with monoclonal antibodies. The neoplastic T cells in all patients had an OKT1+4+8–11+M1-I1–3A1- phenotype, but differed in the reactivity with OKT3. None of the patients' cells exerted helper activity on PWM- induced Ig synthesis. The neoplastic cells of three patients had suppressor activity on PWM-induced Ig synthesis. All patients were positive for human T cell leukemia/lymphoma virus (HTLV) or had antibodies against HTLV antigens. It has previously been shown that the neoplastic cells in Japanese ATLL patients and in patients from the Caribbean are indistinguishable by morphology and marker phenotype. We now show them to be also similar with respect to their functional properties.