Antibodies and assays employing them are taught which are useful for detecting the bulk of mutations which occur in the APC gene in familial adenomatous polyposis. The antibodies are specific for epitopes in the amino terminal or carboxy terminal portion of the protein. A variety of immunoassay formats are described which are all based on the observation that the bulk of the APC mutations which occur in familial adenomatous polyposis and sporadic colorectal carcinomas result in truncated APC proteins. Generally, either the size of APC proteins is determined or the relative binding of amino terminal-binding antibodies to carboxy terminal-binding antibodies is determined.