We have used immunogold staining to locate thrombospondin (TSP) on thrombin-activated human platelets, and have compared its distribution with that of fibrinogen (or fibrin) on thrombin- and ADP-stimulated platelets. To do this, isolated platelets were incubated with monospecific antibodies to TSP or fibrinogen (fib) and the bound IgG located with a second antibody adsorbed to gold particles. Thrombin-induced secretion in Tyrode-Ca2+ was followed by both anti-TSP and anti-fib binding, with large clusters of gold particles observed on the platelet surface. Little or no labeling was observed on unstimulated platelets with either antibody. When secretion was effected in Tyrode-EDTA, anti-TSP IgG still bound to the activated platelets, but the number of particle clusters was significantly reduced. Little binding of anti-fib IgG now occurred. Platelets activated with ADP in the presence of added fib, and subsequently incubated with anti-fib IgG, showed small particle clusters over the whole platelet surface. Thrombin-stimulated platelets from two patients with thrombasthenia bound anti-TSP IgG similarly to normal platelets activated in Tyrode-EDTA. No anti-fib binding occurred. Our results suggest that fib and TSP bind to specific domains on the stimulated platelet membrane. Such sites may be responsible for the mediation of platelet surface contact interactions.