Intravenous injection of lyophilized whole cells of various oral streptococcal strains into muramyldipeptide (MDP)-primed C3H/HeN mice induces rapid anaphylactoid shock. Here we examined the mechanism underlying this shock. In non-primed mice, Streptococcus intermedius K-213K (SiK213) and Streptococcus constellatus T21 (ScT21) produced little or no sign of shock. In MDP-primed mice, SiK213 caused lethal shock, while ScT21 only had a weak effect. SiK213 induced decreases in blood platelets and 5-hydroxytryptamine (5HT) preceding the shock, while the effects of ScT21 were weak. The SiK213-induced 5HT decrease and shock were reduced by a complement-C5 inhibitor. These results suggest that (i) streptococcal bacterial cells can induce rapid platelet responses, (ii) complement-dependent degradation of platelets may be involved in streptococcus-induced shock, (iii) the streptococcus-induced platelet degradation or degranulation may occur largely in the systemic circulation, and (iv) platelets may play a role not only in infectious diseases caused by gram-negative bacteria, but also in diseases caused by gram-positive bacteria.