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We have developed a simple in vivo scintigraphic technique that permits accurate quantitative comparison of intravascular platelet deposition in blood vessels of similar size. Regional count information from scintigraphic images of In-111 platelets and Tc-99m red blood cells is used to determine the ratio of radioactivity in the thrombus relative to that in the blood. Multiplication of this ratio by the percent injected dose (%ID) of In-111 per ml of blood yields a quantity (%ID index) that is directly proportional to the %ID of In-111 platelets in the thrombus as measured in vitro (r = .879, p less than 0.005). This technique is well suited for quantitative studies of scintigraphically detectable platelet deposition in both animals and in man.