An alternative method to radioiodination is outlined for labeling antibodies with a radioactive marker. The method requires the conjugation of the antibody molecule to chelating agents that contain a free amino group and are capable of binding heavy metal ions. Glutaraldehyde is the coupling agent used to bridge the free amino groups on both the chelate and the antibody molecules. 111In is then added to the antibody-chelate conjugate and is bound instantaneously by the chelating portion. Electrophoretic, autoradiographic, immunodiffusion, and hemagglutination techniques were used to confirm the integrity of the 111In-labeled antibodies.