Highly purified alpha-thrombin has been chemically modified in an attempt to determine which features of the molecule are important for normal platelet-thrombin interactions. Modifying agents included diisopropylphosphorofluoridate and 1-chloro-3-tosylamido-7-amino-L-2-heptanone, which modify serine and histidine, respectively, at the catalytic site, as well as N-bromosuccinimide and 2-hydroxy-5-nitrobenzyl bromide, which modify a single tryptophan at or near the fibrinogen-binding site. Active site-directed modification did not appreciably affect the binding characteristics, but prevented platelet activation. In contrast, modification of tryptophan at the macromolecular substrate-binding site resulted in the loss of high affinity binding of thrombin to platelets, while low affinity binding was apparently unaffected. This modification altered but did not abolish the ability of thrombin to effect platelet aggregation and release of [14C]serotonin. These results suggest that residues at the catalytic site are not involved in binding and that the macromolecular substrate-binding site of alpha-thrombin participates in high affinity binding to platelets. These data are also consistent with the existence of at least two types of binding sites for thrombin on the platelet surface as well as more than one platelet-binding region on the thrombin molecule.