Thrombin-induced platelet microbicidal protein (tPMP) is a cationic antimicrobial protein released from platelets by the stimulation with thrombin. tPMP has potent microbicidal activities against a broad spectrum of common microbial pathogens in infective endocarditis. It has been suggested that the resistance of microorganisms to the tPMP-induced killing may be one of the microbial virulence factors that is important in the induction and development of infective endocarditis. We studied the functional tPMP activity against Streptococcus rattus BHT, a tPMP-susceptible strain among the mutans group streptococci. tPMP was prepared by stimulating rabbit platelets with thrombin. Bacillus subtilis 6633 was utilized as an indicator organism highly susceptible to tPMP. B. subtilis 6633 population was subjected to 100% killing by tPMP. The killing of S. rattus BHT was tPMP-concentration and incubation-time dependent. With S. rattus BHT, tPMP bactericidal activity was greater against logarithmic- than stationary-phase cells and incubation temperature dependent with the greater microbicidal activity occurring at $37^{circ}C$ than at lower temperatures. The pH of the reaction mix in tPMP assay did not affect the killing rate for S. rattus BHT. Among 19 oral streptococcal strains tested, only S. rattus BHT was found to be susceptible to tPMP.