A murine monoclonal antibody MLS102 recognizes sialosyl-Tn antigen in mucin and immunohistochemically reacts with more than 80% of colorectal cancer tissues. The purpose of this study was to assess the usefulness of this monoclonal antibody for the immunoscintigraphy of colorectal cancer. Planar and SPECT images were obtained on day 2 or day 3 after injection of 2 mg and 74 MBq 111In-labeled MLS102 antibody into 17 patients with colorectal cancer. Nine of 11 primary tumors and 4 of 6 locally recurrent tumors were detected. Positive images were obtained in all tumors larger than 4.5 x 2.7 cm. Three tumors of less than 2.5 cm and 1 recurrent tumor, which was missed by other imaging modalities, were negative. There were no adverse reactions. Human anti-(mouse Ig) antibody developed in 4 patients. Although improvement of detectability for smaller tumors needs to be pursued, the antibody MLS102 is potentially promising for use in immunoscintigraphy of colorectal cancer.