In two-step targeting technique using bifunctional antibodies, a nonradiolabeled immunoconjugate with slow uptake kinetics (several days) is initially injected, followed by a small radiolabeled hapten with fast kinetics (several hours) that binds to the bispecific immunoconjugate already taken up by the tumor target. In patients with colorectal or medullary thyroid cancer, clinical studies performed with an anti-CEA/anti-DTPA-indium bifunctional antibody and an indium-111-labeled di-DTPA-TL bivalent hapten showed that tumor uptake was not modified compared to results for F(ab')2 fragments of the same anti-CEA antibody directly labeled with indium-111, whereas the radioactivity of normal tissues was significantly reduced (3- to 6-fold). The fast tumor uptake kinetics (several hours) and high or very high tumor-to-normal tissue ratios obtained with the bifunctional antibody technique are favorable parameters for efficient radioimmunotherapy.