In order to selectively, qualitatively or/and quantitatively detect the pyruvate kinase isoenzyme of the tumor M2-PK type (tumor M2-PK) which serves as a tumor marker in human and animal feces to detect a malignant process in the gastrointestinal tract, tumor M2-PK is detected in a stool specimen by an immunoassay technique with the aid of at least one antibody which specifically binds tumor M2-PK and does not cross-react with any other pyruvate kinase isoenzyme. A test kit for diagnosing malignant tumor growth in the gastrointestinal tract and in particular in the intestines contains at least one receptor for tumor M2-PK which does not cross-react with any other pyruvate kinase isoenzyme and optionally contains additional reagents that are necessary for carrying out an immunoassay.