The glycoprotein IIb-IIIa complex (GP IIb-IIIa) is a multifunctional transmembrane protein on platelets. Its most completely described function is as a fibrinogen receptor that mediates platelet aggregation, but it is also involved in clot retraction, signal transduction, calcium transport, and other events. However, the mechanisms that regulate the functions of GP IIb-IIIa during platelet activation are largely unknown. One possible mechanism is phosphorylation, since several other receptors are regulated by this process. We found that GP IIIa, but not GP IIb, was phosphorylated in 32P-labeled platelets, predominantly on threonine residues. Furthermore, GP IIIa phosphorylation increased four-fold in platelets activated with thrombin or phorbol 12-myristate 13-acetate, but not at all in platelets treated with prostacyclin, an inhibitor of platelet activation. The thrombin-induced increase in phosphorylation was inhibited by pretreating platelets with prostacyclin or with staurosporin, a specific protein kinase C inhibitor. Thus, there is an increase in the level or turnover of phosphate on GP IIIa during platelet activation, most likely involving protein kinase C. This phosphorylation may regulate some aspect(s) of GP IIb-IIIa function.