A clonal CD4-positive T-cell line established from the blood of a patient with Sezary syndrome. The reported inability to establish long-term T-cell lines from the blood of cutaneous T-cell lymphoma patients with circulating neoplastic T cells has hindered the development of an in vitro system to investigate Sezary syndrome. We have established a rapidly proliferating T-cell line from the peripheral blood of a patient with Sezary syndrome, which expresses a mature helper T-cell phenotype and contains cytogenetic abnormalities and T-cell receptor gene rearrangements identical to those in the patient's blood. The method of establishment and characteristics of this line are described. 