The SH2 domain-containing inositol 5-phosphatase, SHIP, known to dephosphorylate inositol 1,3,4,5-tetrakisphosphate and phosphatidylinositol 3,4,5-trisphosphate has recently been shown to be expressed in a variety of hemopoietic cells. This 145-kDa protein is induced to associate with Shc by multiple cytokines and may play an important role in the negative regulation of immunocompetent cells mediated by FcgammaRIIB receptor. We report here that SHIP is present in human blood platelets and may be involved in platelet activation evoked by thrombin. Platelet SHIP was identified by Western blotting as a single 145-kDa protein. Both phosphatidylinositol 3,4,5-trisphosphate and inositol 1,3,4, 5-tetrakisphosphate 5-phosphatase activities could be demonstrated in anti-SHIP immunoprecipitates of platelet lysate. Thrombin stimulation induced a tyrosine phosphorylation of SHIP, this effect being prevented if platelets were not shaken or if RGD-containing peptides were present, indicating an aggregation-dependent, integrin-mediated event. Moreover, although the intrinsic phosphatase activity of SHIP did not appear to be significantly increased, tyrosine-phosphorylated SHIP was relocated to the actin cytoskeleton upon activation in an aggregation- and integrin engagement-dependent manner. Finally, the striking correlation observed between phosphatidylinositol 3,4-bisphosphate production and the tyrosine phosphorylation of SHIP, as well as its relocation to the cytoskeleton upon thrombin stimulation, suggest a role for SHIP in the aggregation-dependent and GpIIb-IIIa-mediated accumulation of this important phosphoinositide.