Chronic lymphocytic leukaemia (CLL) B cells are clones representing the mature B cell phenotype. On infection with Epstein-Barr virus (EBV) CLL cells express the EB nuclear antigen (EBNA) complex but unlike EBV-infected normal B cells they do not express LMP nor do they proliferate or immortalize. Furthermore, EBV-CLL rapidly die by apoptosis in culture. In the present study we have used the B cell growth factors interleukin 4 and antibodies to CD40 to induce activation and proliferation of EBV-infected CLL cells. Although cell numbers did not significantly increase, apoptosis was partially inhibited in CLL cells which expressed increased levels of CD23 and were activated to immunoglobulin-secreting lymphoblasts. Expression of LMP was induced by interleukin (IL)-4 and anti-CD40 in all five EBV-infected CLL samples examined. However, this did not enhance cell proliferation or induce immortalization. Further analysis showed that LMP could be detected 4-5 days after EBV infection, and that both IL-4 and anti-CD40 could independently induce LMP but that their effect was additive. These results indicate that LMP expression is dependent on B cell activation processes and that in some circumstances full latent viral gene expression is not sufficient to cause B cell immortalization.