Chronic lymphocytic leukemia of B-cell type (B-CLL) is a malignant disease characterized by monoclonal proliferation of small lymphocytes of B-cell origin, usually associated with suppression of polyclonal B-cell activation (i.e., proliferation and differentiation). Normal human B-cell proliferation is controlled by different T-cell-derived lymphokines, including interleukin 2 (IL2) and gamma interferon (gamma-IFN), that account for the majority of the B-cell growth factor (BCGF) activity produced by mitogen-activated peripheral blood mononuclear cells (PBMCs). We have previously shown an increased and dysregulated secretion of IL2 in peripheral blood from patients with B-CLL. BCGF, IL2, and gamma-IFN productions by phytohemagglutinin (PHA)-stimulated PBMCs were investigated in 13 patients with active untreated B-CLL and 11 healthy donors. B-CLL PBMCs produced a significant amount of BCGF (6 U/ml) despite the low percentage of T cells (10%) associated with this disease as compared with that found in healthy donors (61%). BCGF production in normal controls and B-CLL patients was tripled after irradiation of PBMCs or addition of indomethacin. gamma-IFN secretion in B-CLL patients was decreased when compared with normal controls. Therefore, when gamma-IFN was calculated per fixed number of T cells, production was significantly higher in B-CLL patients than in normal controls, showing a dilution of the productive cells. This study suggests that T cells from B-CLL patients are functional in terms of BCGF production despite their decreased percentage and abnormalities in surface markers.