A substance being studied in the imaging of some types of cancer. M5A is a monoclonal antibody that binds to a protein called CEA on the surface of some tumor cells. It is linked to a radioisotope called indium In 111. The combined substance is injected into the blood, and a machine is used to find cells in the body that bind to it. Indium In 111 anti-CEA monoclonal antibody M5A is a type of radioimmunoconjugate.