The aim of this work was to produce a MoAb able to react with clots but not with fibrinogen. Monoclonal antibodies directed towards DD dimers, against specific products of plasmic digestion of cross-linked fibrin, were obtained. One of these antibodies, F 60/43/8, showed a 1.79 x 10(9) l mol-1 binding constant in spite of the presence of fibrinogen at a 4000 times greater concentration than the cross-linked fibrin. In vitro studies with 125I-F(ab')2 of F 60/43/8 showed that 34-80% of the radioactivity can be found in human clots, in the presence of physiologic concentrations of fibrinogen, and that 96-h washing does not remove the labelled F(ab')2 from the clot. 131I-F(ab')2 was injected into rabbits in which a clot had formed in an artery (six rabbits) or in a vein (six rabbits) of the left ear. Scintigraphic images of the clot were always obtained. In conclusion, the results of this work suggest that F 60/43/8 may be used as a specific antibody for the radioimmunodetection of thrombi.