The integrin GPIIbIIIa is known to be crucial to the formation of platelet aggregates and potentiates adhesion to subendothelial matrices via fibrin(ogen), von Willebrand factor, and vitronectin. Given the demonstration by us and others of widespread platelet aggregation during xenograft rejection, we hypothesized that platelet thrombi might contribute to graft dysfunction during development of hyperacute rejection (HAR), as well as during what we have termed delayed xenograft rejection (DXR), e.g., as seen in complement-depleted rat recipients of guinea pig cardiac xenografts. We therefore tested the effects of a specific GPIIbIIIa antagonist (SDZ GPI 562) during xenograft rejection. Lewis rats received heterotopic guinea pig cardiac xenografts and were treated with GPI 562 alone (HAR model) or in combination with cobra venom factor (CVF) (DXR model). A high (0.5 mg/kg) or a low dose (0.1 mg/kg) of GPI 562 was administered perioperatively and then given twice daily in the same dose until rejection. CVF was given daily until rejection. Plasma drawn after the first dose of GPI 562 and at the time of rejection was tested for the ability to inhibit ADP-stimulated platelet aggregation in vitro. Rejected grafts were analyzed by immunohistology. Plasma from animals in the high-dose group completely inhibited platelet aggregation in vitro, whereas plasma from the low-dose group resulted in only partial inhibition. Similarly, whereas low-dose GPI 562 failed to prolong graft survival, high-dose GPI 562 showed a statistically significant increase in graft survival in both HAR and DXR groups. Immunohistologic studies of HAR showed little effect of GPI 562 on platelet aggregation or activation and no effect on fibrin deposition. However, the combination of high-dose GPI 562 and CVF resulted in a significant decrease in intragraft platelet aggregation, P-selectin expression, and leukocyte infiltration compared with CVF alone. In conclusion, GPIIbIIIa antagonist therapy can inhibit platelet aggregation in vitro and prolong xenograft survival. The diminution of intragraft platelet microthrombi formation and leukocyte infiltration suggests an important role for platelet-dependent mechanisms in leukocyte recruitment during DXR.