The CD5 molecule is expressed by most T cells but it is present on a minor B cell subset. Whilst several studies have provided information on the physiological role of T cell CD5, the functional role of CD5 on B lymphocytes remains unclear. To address this question, tonsillar CD5+ B cells were sorted by dual-colour fluorescence and FACS. Sorted cells were stimulated with polyclonal anti-IgM antibodies (Ab), and monoclonal (MoAb) F(ab')2 fragments of anti-CD5. Proliferative responses were evaluated by enumeration of Ki-67 positive cells using quantitative flow cytometry. Co-stimulation with anti-CD5 MoAb for 3 days did not affect the anti-IgM and IL2-induced proliferation of CD5+ B cells. This was seen under conditions where the anti-CD5 was soluble, adsorbed to the microwells or cross-linked by anti-mouse antibodies. Fewer CD25+ cells were detected, however, in the presence of anti-CD5. In contrast, the proliferative response of CD5+ B cells prestimulated for 3 days with IL-2 and anti-IgM, was sustained in a further 3-day culture period when anti-CD5 was added. It is concluded that CD5 occupancy might provide an additional signal to activated CD5+ B cells favouring their proliferation and differentiation into autoantibody secreting cells.