Platelet adhesion to fibrinogen is important for platelet aggregation and thrombus growth. In this study we have examined the mechanisms regulating platelet adhesion on immobilized fibrinogen under static and shear conditions. We demonstrate that integrin ␣ IIb  3 engagement of immobilized fibrinogen is sufficient to induce an oscillatory calcium response, necessary for lamellipodial formation and platelet spreading. Released ADP increases the proportion of platelets exhibiting a cytosolic calcium response but is not essential for calcium signaling or lamellipodial extension. Pretreating platelets with the Src kinase inhibitor PP2, the inositol 1,4,5-trisphosphate (IP 3 ) receptor antagonist 2-aminoethoxydiphenyl borate (APB-2), or the phospholipase C (PLC) inhibitor U73122 abolished calcium signaling and platelet spreading, suggesting a major role for Src kinase-regulated PLC isoforms in these processes. Analysis of PLC␥2 ؊/؊ mouse platelets revealed a major role for this isoform in regulating cytosolic calcium flux and platelet spreading on fibrinogen. Under flow conditions, platelets derived from PLC␥2 ؊/؊ mice formed less stable adhesive interactions with fibrinogen, particularly in the presence of ADP antagonists. Our studies define an important role for PLC␥2 in integrin ␣ IIb  3 -dependent calcium flux, necessary for stable platelet adhesion and spreading on fibrinogen. Furthermore, they establish an important cooperative signaling role for PLC␥2 and ADP in regulating platelet adhesion efficiency on fibrinogen.Platelet adhesion and aggregation at sites of vascular injury are critical for the arrest of bleeding in traumatized vessels and also for the development of arterial thrombi, precipitating diseases such as acute myocardial infarction and stroke. The two principle adhesive ligands promoting platelet aggregation are von Willebrand factor (vWf) 1 and fibrinogen, with each having distinct, complementary roles in this process (1-5). Fibrinogen acts as a bridging molecule between adjacent activated platelets and is the principle adhesive ligand promoting platelet aggregation under low and intermediate shear flow conditions (1, 2). In addition, it also contributes to stable aggregate formation under high shear (1,5,6). It is well established that the binding of fluid-phase fibrinogen to the platelet surface is dependent on the activation of integrin ␣ IIb  3 (GPIIb-IIIa) through intracellular signaling processes linked to various G protein-coupled and tyrosine kinase-linked receptors (7).Platelets can also adhere to immobilized fibrinogen, a process that is important for primary platelet adhesion onto artificial surfaces, including vascular prostheses (8, 9), and for normal thrombus development (5, 6). In the case of thrombus formation, active integrin ␣ IIb  3 on the surface of firmly adherent platelets adsorbs soluble fibrinogen to the thrombus surface, thereby providing a reactive substrate for the recruitment of additional platelets. In contrast to soluble fibrinogen, the adhesion of platelets onto...