Antibody to platelet factor 4 was used to demonstrate permeation of this factor into the blood vessel wall after endothelial injury in rabbits. The presence of platelet factor 4 antigen in the vessel wall after removal of the endothelium was shown by immunofluorescence 10 and 30 minutes after injury but not 240 minutes afterward. This study demonstrates that factors carried by platelets can enter the vessel wall and that the movement of these platelet products into the vasculature is a short-lived, self-limiting process.