A T lymphoma cell line, KT-3, was established from the peripheral blood of a patient with so-called Lennert's lymphoma when the patient developed leukemic lymphoma. The KT-3 cells stain positively for alpha- naphthyl acetate esterase, PAS, and acid phosphatase. The surface phenotype is E rosette-positive, CD1-, CD2+, CD3-, CD4+, CD5+, CD8-, CD11-, CD20-, CD25 (anti-Tac)+, OKla-1+, HNK-1-, J-5-, and My9-, and the surface membrane immunoglobulin is negative, which is almost the same phenotype as the leukemic cells studied when the culture was started. Southern blot analysis showed rearrangement in both beta and gamma T cell receptor genes. Although KT-3 cells proliferate spontaneously and form clusters, they cease proliferating when they are cultured at low cell densities. They proliferate vigorously in response to macrophages, macrophage-derived factor, or recombinant interleukin 2 (rIL-2) added to the culture. Furthermore, they secrete interferon- gamma (IFN-gamma) spontaneously, and the secretion is augmented when they are stimulated with macrophage-derived factor. The macrophage- derived factor enhancing KT-3 cell growth is different from IL-1 alpha, IL-1 beta, IL-2, or IFN-gamma. To our knowledge, this is the first tumor cell line established from a patient with Lennert's lymphoma. The results conclusively confirmed that, at the least, Lennert's lymphoma included CD4+ T lymphoma and also suggested that cytokines or factors secreted by T lymphoma cells and epithelioid histiocytes play an important role in the histopathogenesis of Lennert's lymphoma.