Blood samples were obtained from the renal vein and a peripheral vein and artery every other day for six days from six canine renal allografts and five renal autografts. Sixteen percent of the platelets brought to the acutely rejecting kidney were retained at two and four days after transplantation. Sequestration of leukocytes was not found. Nor was there evidence obtained for fibrin deposition when fibrinogen was measured in arterial and venous blood. It thus appears that a coagulopathy is not the primary process responsible for the immunologic injury inflicted upon acutely rejected renal transplants. On the other hand, platelet aggregates may contribute to the destruction of allografts by plugging the smaller vessels. These studies provide a rationale for the use of antiplatelet aggregating agents during acute rejection but fail to support the use of anticoagulants.