By using monoclonal B cell targets, cells derived from patients with chronic lymphocytic leukemia, and B cell differentiation factors (BCDF) derived from monoclonal human T cell hybridomas, we have demonstrated marked synergy for differentiation between interleukin 2 (IL 2) and BCDF. IL 2 alone had no effect on the proliferation of differentiation to immunoglobulin secretion in these cell populations; however, in conjunction with a variety of BCDF, differentiation to plaque-forming cells (PFC) was augmented 10- to 100-fold. There was no increase in proliferation as measured by [3H]thymidine incorporation. These effects could be demonstrated with concentrations of IL 2 as low as 5 U/culture, well within the physiologic range, by using either commercially available or recombinant IL 2. The addition of IL 2 to the B cell and BCDF cultures resulted in almost 100% expression of the IL 2 receptor, Tac, on the surface of these cells, and the augmented PFC response could be inhibited 70 to 80% by the addition of anti-Tac to the culture. Kinetic studies revealed that the addition of IL 2 to the B cell cultures could be delayed for up to 72 hr without a change in the PFC response, suggesting that IL 2 was acting as a secondary or synergistic signal for differentiation. Thus, it appears that IL 2 does have a role in B cell maturation mediated, in part, by IL 2 binding to the IL 2 receptor present on certain B cells.