Abstract: Metal radionuclide labeled proteinaceous substances which contain, in their native states, disulfide (cysteine) linkages, which find use in diagnosis and treatment of a variety of pathologic conditions. The radionuclide labeled proteinaceous substances of the present invention are formed by the treatment of the proteinaceous substance of interest with a disulfide reducing agent, followed by reaction of the reduced proteinaceous substance ("H-treated product") with a suitable radionuclide species containing, for example, .sup.99m Tc, .sup.186 Re and .sup.188 Re, or .sup.67 Cu. It has been found that the H-treated products, e.g., antibodies, react specifically with radionuclide species to form stable radionuclide labeled antibodies or other radiolabeled products. The H-treated products may be stabilized or modified in a variety of ways prior to and subsequent to complexation with the metal radionuclide.