Publisher Summary This chapter describes the humanization of the murine monoclonal antibody BW431/26 that has binding specificity for carcinoembryonic antigen (CEA) and is presently used as murine monoclonal antibodies (MAbs) for the immunoscintigraphy of CEA-producing tumors, such as colorectal, breast, and lung carcinomas. MAbs are important tools in all fields of biology and medicine. A large panel of diagnostic as well as therapeutic MAbs has been developed that are of potential value for in vivo diagnosis and therapy in humans. Human MAbs would obviously eliminate such difficulties, but the technology to produce them will require considerable refinement before it becomes generally applicable. However, standard molecular biology techniques can offer a way out of the dilemma. Working on the level of the immunoglobulin genes, it is possible to recombine variable-region genes of the heavy or light chains with constant-region genes of any desired isotype, and, more importantly, there is a choice between the constant-region genes of different species.