In canine experimental models of femoral arterial and venous thrombosis, the feasibility of thrombus imaging with 99Tcm-labelled monoclonal antibody SZ-51 to an α-granule membrane protein (GMP-140) on the surface of activated platelets was investigated. Both the arterial and venous thrombi were clearly discernible at 2 to 8 h after injection of 99Tcm-SZ-51. The suitable imaging time was 4 to 6 h. These results were confirmed by ex vivo images of thrombosed arteries and veins followed by killing the dogs at 8 h postinjection of 99Tcm-SZ-51. Quantitative imaging analysis showed that the ratios between the thrombus and the opposite vessel were increased strikingly over time. The ratios of thrombus to blood or surrounding muscle were 57.09 ± 12.24 and 177.46 ± 9.00 for arterial thrombi and 6.43 ± 4.70 and 83.22 ± 68.98 for venous thrombi at the time of death. Radiolabelled antibody was cleared from the blood with T1/2α of 16.25 ± 13.39 min and T1/2β of 7.28 ± 1.15 h. These results show that it is feasible to detect noninvasively thrombus formation in vivo using 99Tcm-labelled monoclonal antibody SZ-51.