A large body of data accumulated over the past several years suggests that the periodontal pathogen Porphyromonas gingivalis is associated with cardiovascular disease. Circulating bacteria may contribute to atherogenesis by promoting CD11b/CD18-mediated interactions between neutrophils and platelets, causing reactive oxygen species (ROS) production and aggregation. We have previously demonstrated that P. gingivalis induces aggregation and ROS production in whole blood, and that the anti-inflammatory mediator lipoxin A4 (LXA4) inhibits these responses by modulating platelet-neutrophil interaction through a down-regulation of the bacterium-induced surface expression of CD11b/CD18 on neutrophils, likely by inhibiting Rac2 and Cdc42 signaling pathways. Furthermore, P. gingivalis, unlike other periodontopathic bacteria, has been shown to trigger platelet aggregation, mainly through the interaction between bacterial gingipains and protease-activating receptors (PARs) on the platelets. Since platelet aggregation precedes thromboembolic events, this is an important pathogenic feature of the bacterium. The aim of this study was to investigate the effect of gingipains on P. gingivalis-induced cell activation in whole blood. Platelet/leukocyte aggregation and ROS production was examined by lumiaggregometry. This study shows that leupeptin, a protease inhibitor of gingipains, inhibits P. gingivalis-induced aggregation and ROS production in whole blood. Supernatants of bacteria suspensions induced no ROS-production, but an aggregatory response that was also inhibited by leupeptin. In conclusion, P. gingivalis-induced aggregation and ROS production in whole blood is mainly dependent on gingipains. However, since bacterial supernatants (containing soluble gingipains) stimulate only aggregation, this suggests that a gingipain/PAR-mediated mechanism in combination with phagocytosis of whole bacterium is a prerequisite for inducing a respiratory burst and an inflammatory response. These findings may contribute to new strategies in the prevention and treatment of periodontitis-induced inflammatory disorders, such as atherosclerosis.