A radioimmunoconjugate of a humanized monoclonal antibody (MoAb) A33 labelled with iodine 124 (I-124). MoAb A33 recognizes A33 antigen, a 43 KDa transmembrane glycoprotein of the immunoglobulin superfamily, highly and homogenously expressed in 95% of colorectal cancer metastases, with only restricted expression in normal colonic mucosa. I-124 MoAb A33 delivers beta particle emitting I-124 nuclide directly to metastatic colorectal tissues, thereby this agent could be used in kinetics studies or radioimmunotherapy.