In situ hybridization with specific RNA radiolabeled probes was used to analyze the production of interleukin-2 (IL-2) and interferon-gamma (IFN-gamma) by 21 low-grade follicular lymphomas (FLs). All of the 21 FLs tested contained lymphokine-synthesizing cells in the interfollicular and follicular areas. Enumeration of lymphokine synthesizing cells indicated heterogeneous IL-2 production among lymph nodes tested; 2 of the 21 had much higher densities of IL-2-producing cells (855 and 570/cm2) than did the remaining 19 (mean, 92 +/- 15/cm2). IFN-gamma-producing cells displayed no such variation (mean, 77 +/- 8 IFN-gamma-producing cells/cm2). The mean IL-2/IFN-gamma-producing cell ratio was 2.69 +/- 0.84, indicating preferential induction of IL-2. The detailed distribution of lymphokine-producing cells showed that IL-2 and IFN-gamma-producing cells were located mainly in the follicular areas. The mean follicular/interfollicular ratio was 1.82 +/- 0.16 for IL-2 and 1.92 +/- 0.19 for IFN-gamma-producing cells. The results show that T-cell activation, defined by lymphokine production, occurs in FL lymph nodes in direct contact with malignant B cells. Thus, lymphokine production may play an important role in the control of tumor growth, which is the result of interaction between tumor cells and host-derived immune reaction.