Because of the high intranuclear toxicity of Auger-electron emitters, the use of radioiodinated (123I, 125I) 5-iodo-2'-deoxyuridine (IUdR)-antibody conjugates for cancer therapy has been examined. The results have demonstrated that all the conditions necessary for labeling DNA in vivo are present: uptake of the radiolabeled immunoglobulin by target cells, its subsequent internalization, the degradation of the IUdR-protein conjugate by lysosomal enzymes, and the incorporation of the radionucleoside into DNA.