B and T cell function were studied in 10 patients with active Behçet's disease (BD) and in 10 normal subjects. Peripheral B lymphocytes infected with Epstein-Barr virus (EBV) were cultured for 20 days in the presence or absence of autologous T cells. Immunoglobulin M and G secretions into the supernatants were assessed with an enzyme-linked immunosorbent assay. The extent of suppression of EBV-induced B cell activation by autologous T cells was significantly decreased in active BD patients as compared to normal subjects at a T:B ratio of 1:1, whereas the suppression ratio was in the normal range at a T:B ratio of 4:1. The IgM and IgG secretions in purified B cell cultures were significantly higher in active BD patients as compared to control subjects. Thus, an increased B cell function associated with a defective EBV-specific T cell suppressive function could explain at least in part the immunological disorders in BD patients.