A T lymphoma cell line (KT-3) established from a patient with Lennert's lymphoma showed macrophage-dependent growth. Macrophage-derived factors were able to replace the macrophage functions. Experiments using a variety of cytokines demonstrated that KT-3 proliferated in response to recombinant interleukin-2 (rIL-2), rIL-4, or rIL-6 but did not proliferate in response to rIL-1 alpha, rIL-1 beta, rIL-3, recombinant granulocyte colony-stimulating factor (rG-CSF), rGM-CSF, recombinant interferon-alpha (rIFN-alpha), rIFN-gamma, recombinant tumor necrosis factor (rTNF-alpha), or native IFN-beta. Polyclonal rabbit anti-IL-6 antibody almost completely neutralized the activities of macrophage-derived factors or IL-6 but not IL-2 or IL-4. Scatchard plot analysis demonstrated that KT-3 cells indeed express IL-6 receptors. The results indicate that the macrophage-derived factor that supports the growth of KT-3 is IL-6 and suggest that macrophage-derived IL-6 may play an important role in the histopathogenesis of Lennert's lymphoma.