Incorporation of indium-111 labeled platelets (In-111-P) into venous thrombi and pulmonary emboli may permit rapid detection of these thromboemboli by gamma imaging. In a series of dogs in which femoral-vein thromboses and/or pulmonary embolism were induced experimentally by stasis and small amounts of thrombin, several questions pertinent to the sensitivity, specificity, and potential applicability of this approach were addressed. When In-111-P were injected i.v. before thrombus induction or embolus release, femoral-thrombus images were consistently detectable within 15 min, whereas control femoral-vein images were unremarkable. Pulmonary emboli were also promptly imaged, and such In-111-P images agreed well with defects on 99mTc MAA perfusion scans. When thrombi were aged in vivo for up to 10 h after formation, they could still be imaged within 20-90 min after In-111-P injection. Administration of heparin, as an initial bolus followed by constant infusion, blocked platelet deposition on femoral-vein thrombi as assessed by both thrombus:blood ratios and failure to image. Injection of protamine at 6 h resulted in prompt thrombus imaging. Apparently, this approach may well have applicability to the detection of thromboemboli in humans, since imaging remains possible in canine thrombi aged in vivo for 10 h so long as heparin therapy has not been instituted. The dose of heparin required to inhibit imaging is not known. If these data prove comparable in humans, they suggest that imaging of thromboemboli could be achieved so promptly that only modest delay in the institution of heparin therapy would be required.