OBJECTIVE Antigen-driven B-cell proliferation and maturation occur in germinal centres present in lymphoid tissues. This process is highly dependent on functional interactions between B and T lymphocytes. In vitro activation of CD40 present on B cells mimics B cell-T interactions and allows the proliferation of normal Epstein-Barr virus (EBV)-negative B lymphocytes. In HIV-1-seropositive individuals, B cells become exposed to free viral particles and to infected T lymphocytes while migrating through germinal centres. The effect of HIV-1 viral exposure on CD40-activated B lymphocytes was therefore examined. METHODS Freshly isolated B lymphocytes were cultured in vitro through activation of CD40. B-cell proliferation, HIV-1 infectivity and viral production were monitored following B-lymphocyte exposure to HIV-1. In addition, HIV-mediated fusion between infected B cells and uninfected CD4+ T lymphocytes was assessed in a coculture assay. RESULTS EBV-negative, CD40-activated human B lymphocytes were directly infected by HIV-1. The infection significantly reduced their proliferation rate. Viral production was detected in B-cell culture supernatant. Numerous fusion events indicated that HIV-1 infection of B lymphocytes could spread to T lymphocytes following HIV-1-mediated fusion of these two cell types. CONCLUSIONS In view of the importance of B cell-T cell interactions in the maintenance of a functional immune system, disruption of B-lymphocyte development could have direct implications on the course of AIDS progression.