Immune reactions against microorganisms play an important pathogenic role in Adamantiades-Behçet's disease. We had previously obtained Streptococcus sanguinis (strain BD113-20), isolated from the oral cavity of patients with Adamantiades-Behçet's syndrome. To investigate the pathogenesis of this isolate, we examined neutrophil reactions and levels of cytokine production by lymphocytes after stimulation with the strain. The reactions of neutrophils were examined by chemiluminescence assay using whole blood. The amounts of interferon gamma (IFN-gamma) and interleukin (IL)-4, IL-8, IL-10 and IL-12, produced by peripheral blood mononuclear cells, were measured by ELISA. Strain BD113-20 activated neutrophils from Adamantiades-Behçet's patients and healthy volunteers, and, in addition it increased the IFN-gamma production by lymphocytes. Lymphocytes from Adamantiades-Behçet's patients showed a dominant T helper-1 immune response. Results indicated that both bacterial stimulation and host hypersensitivity might be involved in the symptoms and pathogenesis of Adamantiades-Behçet's disease.