Human platelet cytosol contains several phosphoinositide-specific phospholipases C (PLCs) separable by stepwize ion-exchange chromatographies [Banno, Yu, Nakashima, Homma, Takenawa and Nozawa Biochem Biophys Res Commun 1990 167; 396-401]. In the present study, one type of PLC (cPIP2-PLC) for which phosphatidylinositol 4,5-bisphosphate (PIP2) was the preferred substrate, was isolated from human platelet cytosol as a truncated form (100 kDa). A mPIP2-PLC was purified to near homogeneity from the cholate extract of human platelet membranes. The purified 150-kDa mPIP2-PLC was truncated during purification, and ran as 100-kDa and 45-kDa polypeptides on SDS-polyacrylamide gel electrophoresis. The 100-kDa component of cytosolic PLC (cPIP2-PLC) and the 150-kDa, 100-kDa and 45-kDa polypeptides of mPIP2-PLC were all recognized by the antibody raised against PLC-β. The 150-kDa enzyme was immunoprecipitated by anti-PLC-βantibody from freshly prepared human platelet cytosol. The catalytic properties of the platelet 100-kDa form were observed to be very similar to those of 100-kDa form derived from bovine brain PLC-β. These results indicate thatβ-type PLC exists in both cytosol and membranes of human platelets.