 When colorectal cancer has spread to other organs, it is generally considered incurable but with a limited number of treatment options. Colorectal cancer cells express proteins on their surface known as antigens, and one of these is called the A33 antigen. An antibody which targets the A33 antigen was initially developed in the mouse and found to bind to human colorectal cancer cells. Because humans developed immune reactions when given the mouse antibody, an antibody, which is more like normal human antibodies, was developed humanised A33 antibody. In order to increase its effectiveness, radioactive iodine 131I has been attached to the antibody so that the antibody can deliver radiation directly to colorectal cancer cells. Previous studies have shown that both the unlabelled humanised A33 antibody as well as the humanised A33 antibody tagged with radioactive iodine can be administered safely to humans with no major allergic reactions. The addition of chemotherapy to radiolabelled 131I-huA33 may result in a treatment that is more effective for the treatment of colorectal cancer than either agent alone. 