The binding of viridans group streptococci with human platelets was analyzed by two-color flow cytometry. Binding was detected within 15 s of mixing bacteria and platelets. At ratios of bacteria to platelets of 1:1, 10:1, 100:1, and 1,000:1, the percentages of bound streptococci (mean +/- standard deviation) were 93.2% +/- 5.4%, 80.0% +/- 8.6%, 39.8% +/- 11.1%, and 12.5% +/- 2.0%, respectively. Binding of labeled bacteria was reversed by adding a 500-fold excess of unlabeled streptococci. These results demonstrate that streptococcus-platelet binding is rapid, reversible, and saturable, which suggests a specific receptor-ligand interaction.