The interaction of isolated and radioiodinated thrombospondin with washed human platelets has been characterized. The ligand bound to nonstimulated and thrombin-stimulated platelets in a time-dependent manner, and apparent steady state was reached within 25 min. Binding was not due to iodination of the ligand and was inhibited by nonlabeled thrombospondin but not by unrelated proteins, and bound ligand was identical with thrombospondin in terms of subunit structure. Nonlinear curve-fitting analyses of binding to resting platelets suggested the presence of a single class of sites which bound 3,100 +/- 1,000 molecules/platelet with an apparent Kd of 50 +/- 20 nM. This interaction was not attributable to contaminating cells or inadvertant platelet activation. Binding to thrombin-stimulated platelets had a lower apparent affinity (Kd = 250 +/- 100 nM) and higher apparent capacity (35,600 +/- 9,600 molecules/platelet). Thrombin-enhanced binding was dependent upon agonist dose and platelet stimulation. Fibrinogen, a monoclonal antibody to GPIIb-IIIa, temperature, and divalent ions had differential effects upon thrombospondin binding to resting and stimulated platelets, suggesting the presence of two distinct mechanisms of thrombospondin binding to platelets. While thrombospondin binding to thrombin-stimulated platelets occurs with characteristics similar to those observed for fibrinogen, fibronectin, and von Willebrand Factor, its high affinity interaction with resting platelets is unique to this adhesive glycoprotein.