Imaging with the 99mTc-T2G1s monoclonal antifibrin antibody fragment (Fab') has demonstrated promise in the noninvasive detection of venous thrombi in humans. The purpose of this study was to determine whether chronic arterial thrombi can also be detected by antifibrin antibody imaging. METHODS Eighteen subjects with chronic arterial thrombi were studied with planar and tomographic imaging at 0 to 24 hr postinjection of 99mTc-labeled T2G1s monoclonal antifibrin antibody fragment. Imaging with 111In-labeled platelets was also performed. Images were visually graded by two observers as 0, 1, 2 or 3 (no, faint, moderate or marked) uptake, and quantitative analysis of tomographic images was done in 13 subjects. RESULTS On visual analysis of planar images, 44% (8 of 18) of antifibrin patient studies were 1.0 or more and 66% (10 of 18) were judged negative compared with 94% (15 of 16) of platelet patient studies judged 1.0 or more and 6% (1 of 16) judged as negative (p < 0.01). Visual analysis of tomographic images was similar, with 61% (11 of 18) of antifibrin studies graded 1.0 or more compared with 100% (17 of 17) of platelet studies (p < 0.01). The tomographic target-to-background ratio was higher with platelets than with antifibrin antibody (2.5 +/- 1.4 versus 1.8 +/- 1.0, p < 0.05). CONCLUSIONS In the large-vessel chronic arterial thrombi studied, the results of 99mTc-labeled monoclonal T2G1s antifibrin Fab' imaging were positive in only one-half of the patients studied, significantly less than the findings with platelet imaging, which were positive in all subjects. The higher rate of positive images with labeled platelets than with labeled antifibrin antibodies may be largely due to thrombus age, with continued platelet deposition but little active fibrin deposition.