Technetium-99 m (99mTc)-labelled conjugates of an anti-fibrin monoclonal antibody, DD-3B6/22, have been assessed for their detection of vascular thrombi in a rabbit model. DD-3B6/22 binds to a D-dimer epitope present on cross-linked fibrin but absent from the fibrin monomer or fibrinogen. Injection of a 99mTc-labelled Fab' fragment of DD-3B6/22 allowed delineation of model thrombi as early as 30 min postinjection (p.i.) with optimal localization at 4-5 h. Thrombus label uptake at 4 h p.i. was 0.304 +/- 0.106% injected dose/g (% ID/g) compared with 0.022 +/- 0.001% ID/g after the injection of a control Fab' fragment. These results suggest that the 99mTc-labelled Fab' fragment of DD-3B6/22 has excellent potential for scintigraphic detection of vascular thrombi in humans.