In order to improve cancer imaging with radiolabeled antibodies, three factors appeared to be of particular importance: (1) The selection of the most favorable monoclonal antibody directed to tumor-associated antigen. (2) The use of F(ab')2 or Fab fragments. (3) Selection of the most convenient isotope. Monoclonal antibody CEA 102 was produced by immunization by purified CEA, and its F(ab')2 fragments were compared with whole IgG as a radiotracer for radioimmunodetection of the colorectal cancer. Fragments were eliminated from the circulation twice as fast as whole IgG, and tumor-to-background ratio was achieved more than 1 at 2-3 days with F(ab')2, but 6-7 days with whole IgG in tumor bearing nude mice. In clinical study, F(ab')2 demonstrated clear image on the 1 at day after injection, whereas achievement of the image was possible on the 3rd day in whole IgG. These results indicated that fragments are preferred over whole IgG. Therefore fragments make it possible to preclude dual isotope subtraction methods, and omit the long delays before imaging. They also make it possible to use short half life radionuclides with excellent photon properties, such as 123I and 99mTc.