The ability of endocarditis and dental strains of Streptococcus sanguis to induce platelet aggregation in plasma (PRP) from normal subjects were examined and compared to responses of PRP with known platelet membrane glycoprotein (GP) and response defects. S. sanguis strains differed in their ability to induce normal PRPs to aggregate. Strains that induced PRP aggregation in more than 60% of donors were significantly faster agonists (mean lag times to onset of aggregation less than 6 min) than those strains inducing response in PRPs of fewer than 60% of donors. Platelets from patients with Bernard-Soulier syndrome aggregated in response to strains of S. sanguis. In contrast, platelets from patients with Glanzmann's thrombasthenia and from a patient with a specific defect in response to collagen were unresponsive to S. sanguis. These observations show that GPIb and V are not essential, but GPIIb-IIIa and GPIa are important in the platelet response mechanism to S. sanguis. Indeed, the data suggests that the platelet interaction mechanisms of S. sanguis and collagen may be similar.