The protein kinase C (PKC) was secreted from thrombin-stimulated human platelets in a time- and dose-dependent manner. The PKC specific inhibitors Ro31-8220 (0.05 muM) and GF 109203X (0.5 muM) totally inhibited the secreted kinase activity. Western blot analysis of the secretory components showed reactivity to PKC alpha, PRC beta II, and PKC delta antibodies, but not to PKC betaI, and p42/44 MAPK, although they were present in lysed platelets. The fractionation of platelets secreted components showed that PHC activity increased in both soluble and microparticle fractions after thrombin treatments. This is the first report demonstrating that activated human platelets selectively secrete protein kinase C isozymes. Protein kinase C secreted by platelets in this unique manner may have an extracellular role in the plasma, and may regulate cellular functions, including remodeling of vascular endothelial cells.