A new method is described for labelling proteins with 99Tcm. Labelling of fibrinogen resulted in a radiochemical yield of about 80%. After clotting this 99Tcm-labelled fibrinogen with trombin, an equal percentage of the radioactivity was found in the clot illustrating the retained biological behaviour of this labile protein after our labelling procedure. Labelling of a monoclonal antibody (MoAb) directed against fibrin resulted in a labelling percentage of about 70%. Immunoreactivity of this antifibrin (Y22) is hardly affected by the procedure as demonstrated by a double-sandwich enzyme immunoassay (EIA). Perfusion experiments on a gamma camera in which a plasma clot in a glass chamber was perfused with plasma containing 99Tcm-labelled Y22 revealed an excellent uptake of activity by the clot within 2 h. Finally an animal experiment is presented which showed clear visualization of the thrombi in the jugular vein and the abdomen of a rabbit by scintigraphy after the administration of 99Tcm-labelled antifibrin (Y22).