Biosynthetically-radiolabeled MoAbs provide a tool to test whether structural modifications of the MoAbs influence the results of conventional immunoscintigraphy. When biosynthetically-labeled 75Se-MoAbs from the Mel-14 hybridoma were injected into mice with melanoma xenografts, the high tumor recovery supported the hypothesis of a structural advantage. The increased excretion of 75Se obtained by supplementing the diet of the mice with cold selenium did not reduce the tumor recovery, demonstrating an accumulation of the free radionuclide in normal tissue.