A protein kinase activity analogous to that found in interferon-treated HeLa cells is detectable in human plasma rich in platelets. This kinase activity is manifested by the phosphorylation of an endogenous M r 72 000 protein which could be conveniently assayed after partial purification on poly(G)—Sepharose. Here, we show that the protein kinase system in the plasma consists of at least 2 components. The protein kinase is found to be localised in the platelet whereas most of the substrate (the M r 72 000 protein) is found free in the plasma and a fraction of it associated with the surface of platelets.
