Heparin inhibits platelet function and can cause thrombocytopenia. In an effort to understand these phenomena, we have measured the binding of (3H)-heparin to resting and stimulated platelets. In platelet-rich plasma, a single class of saturable heparin binding sites was observed (apparent dissociation constant [kd] approximately 0.55 microgram/mL, R approximately 0.059 microgram/10(9) cells). In gel-filtered platelets, a similar class of sites was present but with a greater binding capacity (apparent kd approximately 0.56 microgram/mL, R approximately 0.44 microgram/10(9) cells). Gel-filtered platelets that had been stimulated with thrombin displayed two classes of binding sites: a high- affinity class (apparent kd1 approximately 1.1 microgram/mL, R1 approximately 0.39 microgram/10(9) cells) corresponding to that of the unstimulated cells, and a low-affinity class (apparent kd2 approximately 13 micrograms/mL, R2 approximately 2.2 micrograms/10(9) cells). Heparin binding was also increased in platelet-rich plasma when the cells had been stimulated by adenosine diphosphate (ADP) to release, but not when ADP caused primary aggregation without release. Binding was not dependent on extracellular calcium, nor was it reduced by monoclonal antibodies to platelet membrane glycoproteins Ia/IIa, Ib, IIb/IIIa, or IV. Because the apparent dissociation constant of the high- affinity sites (approximately 0.55 microgram/mL) falls in the range of heparin concentrations achieved clinically, these binding sites may be involved in the platelet dysfunction and immune-mediated thrombocytopenia associated with therapeutic heparin. The low-affinity, high-capacity class of sites, which appears after cell stimulation, may participate in the process of platelet adhesion.