Black-pigmented bacteria which produce cytotoxic metabolic end-products and cell membrane-associated proteases have been reported to play an important role in the pathogenesis of periodontal diseases. These bacterial virulence factors can be modified by the environmental conditions including nutrients supplied variously into the oral cavity. Although glucose is one of the most essential nutrients for oral bacteria, the exogenous supply of glucose may be discontinuous and the glucose concentration in a periodontal pocket may be influenced by the depth of the periodontal pocket. Therefore, effects of glucose as an environmental factor on the virulence factors of Prevotella intermedia, Prevotella nigrescens and Porphyromonas in the presence o glucose.,bo t P. intermedia and P. nigrescens markedly decreased the production of cytotoxic end-products including succinate.,isobutyrate,isovalerate and ammonia, although their growth was increased. Furthermore, the proteolytic activities such as immunoglobulin- albumin- and casein-degrading activities of these bacteria were decreased in the presence of glucose. On the other hand, no effect of glucose on the metabolic activity of P gingivalis was observed. These results suggest that pathogenicity of P. intermedia P. nigrescens may be decreased by the presence of glucose.