Two I-131 labeled monoclonal antibodies that react specifically with human gastrointestinal cancers in cell cultures were administered to 90 cancer patients for the scintigraphic detection of cancer sites. Antibody 17-1A, or its F(ab')2 fragments, accumulated significantly in 27 of 46 (59%) colorectal cancer sites, but not in 21 nonepitheliomatous colon cancers and cancers at other sites. Antibody 19-9, or its F(ab')2 fragments, showed significant accumulation in 19 out of 29 (66%) colorectal cancer sites. In 17 patients, immunoscintigraphy with antibody 19-9 correlated with an immunoperoxidase study with the same antibody on resected tissue specimens. In 12 patients injected with two antibodies (17-1A + 19-9, or anti-CEA + 19-9), ten of 13 colorectal cancer sites were positive.