Recent studies performed in the laboratory have established that interleukin-4 (IL-4) used in combination with anti-CD40 monoclonal antibody (MoAb) 89 presented on Ltk- mouse fibroblasts stably expressing human Fc gamma RII/CDw32 (referred to as the CD40 system) sustains long-term proliferation of normal human B cells. In the present study, B-cell chronic lymphocytic leukemias (B-CLLs) activated through slgs or CD40 were examined for their capacity to proliferate and differentiate in response to various cytokines. Our results indicate that the outcome of IL-4 stimulation on the in vitro growth of B-CLL depends on the signalling pathway used for their activation. Whereas IL-4 did not display any growth-stimulatory effect on B-CLL activated by Ig cross-linking agents, it could stimulate DNA synthesis and enhance the viable cell recovery when leukemic B cells were cultured in the CD40 system. Most B-CLL samples were induced for IgM synthesis upon Staphylococcus aureus strain Cowan I stimulation. This Ig response was potentiated by IL-2 and antagonized by IL-4. Anti-CD40 MoAb used alone or in combination with cytokines (IL-1 alpha to IL-6, interferon gamma, tumor necrosis factor gamma, and transforming growth factor beta) failed to induce Ig secretion from B-CLL cells. No evidence for Ig isotype switching was obtained with the cytokines listed above, regardless of the mode of activation. Taken together, our results suggest that B-CLL cells can be partially released from their apparent maturation block by IL-2 and Ig cross-linking agents. In contrast, combinations of IL-4 and cross-linked anti-CD40 antibodies induced entry of B-CLL cell into cycle, but poorly stimulated their differentiation into Ig secreting cells.