Expanded populations of T lymphocytes bearing γδ T cell receptors have been detected in several patients with Felty syndrome. The goal of this study was to functionally characterize these Iymphocytes in a newly described patient with this disease. For this, fluorescence-activated cell sorter analysis of T cell surface antigens, proliferation, and tumor necrosis factor α enzyme-linked immunosorbent assays, as well as quantitative assays of immunoglobulins secreted by pokeweed mitogen-driven B cells were performed. The γδ cells, that expressed a CD3+CD4-Vγ9-Vδ2+Cδ+ phenotype, and constituted 60% of the peripheral blood T cells, did not proliferate after triggering with anti-CD3, but did secrete tumor necrosis factor α and the addition of these cells to pokeweed mitogen-stimulated B cells from the patient decreased their secretion of immunoglobulin M while augmenting IgG secretion. These data suggest that the expanded anergic Vγ9-Vδ2+ γδ cells can play an immunoregulatory role in the patient.