One proposed mechanism in the pathogenesis of infective endocarditis is the direct aggregation of platelets by the bacteria causing the disease. Some, but not all, strains of Streptococcus sanguis have been reported to aggregate platelets but the taxonomy of this and related taxa has changed recently. The ability to aggregate platelets by 24 genetically grouped laboratory stock strains was studied along with 8 recent isolates from cases of endocarditis. Strains belonging to S. sanguis could aggregate platelets, but not S. gordonii, "S. parasanguis", S. mitis, S. oralis or related taxa. Also, preliminary data indicate that certain biotypes of S. sanguis lack the ability to aggregate platelets. Of the recent clinical isolates, only 4 aggregated platelets and each of these showed phenotypes typical of S. sanguis. These data suggest that the ability to aggregate platelets is not essential for an organism to be able to cause endocarditis, although it may be a significant virulence factor.