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A simple, noninvasive method for the direct visualization of pulmonary emboli would be of considerable value for the diagnosis of this common disorder, especially in patients who have underlying parenchymal lung disease or are too ill to undergo pulmonary angiography. For this purpose, we have investigated the accumulation of In-111 platelets in acute pulmonary emboli. Radiolabeled venous thrombi were produced in 6 dogs by the injection of human thrombin and Tc-99m sulfur colloid into occluded segments of both jugular veins. One hour later, the thrombi were released and gamma camera images demonstrating the position of the Tc-99m labeled pulmonary emboli were obtained. Autologous platelets labeled with In-111 oxine were then injected peripherally and sequential images were obtained for one hour. The dogs were then sacrificed, and the emboli and tissue samples were removed and assayed for activity. Sixteen pulmonary emboli containing Tc-99m sulfur colloid were seen by imaging and 14 of these were detected with In-111 platelets; 5 emboli were visualized immediately and uptake in the remaining 9 appeared on later images. In all animals, at least one embolus was detected by imaging. Mean embolus weight was 538 mg, mean In-111 uptake was 1.09% dose/g embolus, and mean embolus/blood ratio was 16.1. In 2 animals, images showed additional foci of increased In-111 uptake, distal to an embolus containing Tc-99m; this uptake may reflect thrombus propagation. Our results demonstrate that acute pulmonary emboli in dogs can be readily detected by imaging with In-111 labeled platelets, injected after embolization.