Lymphoblasts from the peripheral blood of a 10-year-old boy who was treated for T-cell acute lymphoblastic leukemia were heterotransplanted intraocularly into nude mice. The resultant tumors and their metastases were serially passaged both intraocularly and subcutaneously in the mice. The mouse tumors closely resembled morphologically and cytochemically a human T-cell lymphoma. The primary cells, as well as cells from subsequent in vivo passages, were predominantly of a suppressor T-cell phenotype (OKT8-positive). No viral products were identified. Chromosome analysis revealed a near-diploid karyotype with a translocation between chromosomes 11 and 14. The tumorigenicity, morphology, cytochemistry, immunologic phenotype, and karyotypic pattern of the cells remained constant through six serial in vivo passages over a period of 10 months. This is the first report of direct heterotransplantation and long-term in vivo maintenance of primary human T-cells in immunologically unmanipulated nude mice.