Homogenates of human platelets can mediate the synthesis of phosphatidylinositol from myoinositol and cytidine diphosphate diglyceride. The cytidine diphosphate diglyceride: myoinositol, phosphatidyl transferase activity is particulate-bound, and the highest specific activity is found in the membrane fraction. The production of phosphatidylinositol is decreased by sulfhydryl-binding agents, and the addition of thiols to the platelet homogenates increases the enzymatic activity. The reaction exhibits a pH optimum of 8.5-9.0. Divalent cations stimulate the reaction, and manganous chloride was the most effective of those investigated. The K(m) of the enzyme for myoinositol is 0.27 mM, and the K(m) for cytidine diphosphate diglyceride is 0.53 mM. The enzymatic activity of platelets isolated from patients with several diseases known to interfere with platelet clotpromoting function is similar to the enzymatic activity of platelets from normal donors.