Cyclooxygenase-2 (COX-2) is induced after the activation of cells by a variety of proinflammatory agents. Recently, evidence has shown that COX-2 may play a role in the pathogenesis of pulpal inflammation. However, little is known regarding the mechanism of pulpal inflammation at the site of bacterial infection. The purpose of this study was to determine the effects of the supernatants from black-pigmented Bacteroides (Porphyromonas endodontalis, Porphyromonas gingivalis, and Prevotella intermedia) on the COX-2 expression in primary human pulp cells in vitro. Investigations of the time dependence of COX-2 mRNA expression in black-pigmented Bacteroides-treated human pulp cells revealed a rapid accumulation of the transcript, a significant signal first detectable after 1 h of exposure. In addition, black-pigmented Bacteroides also up-regulated COX-2 protein expression in human pulp cells. Data from our in vitro experiment showed that black-pigmented Bacteroides were capable of stimulating COX-2 expression in human pulp cells. These results indicate that black-pigmented Bacteroides species may play an important role in the pathogenesis of pulpal inflammation. The activation of COX-2 may be one of the distinct host degradative pathways in the pathogenesis of microbial-induced pulpal/periapical inflammation.