The radiolabeling of antibodies is considered in terms of the choice of radionuclide, the method of conjugation, and the effect of conjugation on plasma clearance. Iodination techniques are reviewed, but the major emphasis is placed on the methods of conjugating metallic radionuclides using bifunctional chelating agents. The technique of producing clinically useful radiolabeled antibodies by balancing altered substrate specificity caused by radiolabeling against accelerated plasma clearance is discussed.