Individual leukaemic B cells of chronic lymphocytic leukaemia (CLL) do not proliferate to B cell growth factor (BCGF) or interleukin 2 (IL-2) when co-stimulated with immunoglobulin (Ig) ligands. To exclude possible defective signalling via surface Ig (sIg), phorbol myristate acetate (PMA) plus calcium ionophore (A23187) were used to activate purified CLL B cells and compared with staphylococcal protein A coupled to sepharose beads (Seph-PA). RNA synthesis and phenotypic changes after PMA plus A23187 stimulation indicate that CLL B cells from (10) different individuals are similarly able to undergo the G0 to the G1 phase transition and express surface activation antigens. In contrast, they are variable in the capacity to show DNA synthesis, which occurred in only six out of 10 cases. Even in the presence of BCGF (10%, v/v) or IL-2 (50 U/ml) four out of nine CLL B cells activated with PMA plus A23187 or PMA alone were still unable to proliferate although they were induced to express CD23, 4F2, CD25 and OKT9 antigens by PMA plus A23187. However, PMA plus A23187 induced IgM secretion which increased further in response to IL-2 even in the absence of DNA synthesis. Moreover, in other CLL B cell populations, the unresponsiveness to growth factors upon co-stimulation with Ig ligands (Seph-PA) may simply reflect a defective signalling via sIg cross-linking which can be circumvented by PMA plus A23187 stimulation. Recombinant Interferon-gamma (50 U/ml) failed to affect DNA synthesis and IgM secretion.