The development of the hybridoma technology by Kohler and Milstein 1 for the production of monoclonal antibodies created a new tool for the diagnosis and therapy of cancer. 2 For the determination of in vivo usefulness of (mostly murine) monoclonal antibodies directed against human tumor-associated antigens, relevant tumor systems are needed. Human tumors transplanted to nude mice provide such a model. In previous years, the model has been used successfully in analyzing the efficacy of radiolabeled monoclonal antibodies to localize in tumor lesions. Moreover, conjugates of monoclonal antibodies with radionuclides, drugs, toxins, or enzymes have been evaluated in the nude mouse-human tumor model for therapeutic effectiveness before their introduction into the clinic.