Two cases of highly malignant tumors, one originating in the sigmoid colon and the other in the rectum, are presented. Both tumors showed light microscopic, electron microscopic, and immunohistochemical evidence of multidirectional differentiation. The tumors were composed mainly of undifferentiated cells, but focally merging into areas with adenocarcinomatous and squamous differentiation. Ultrastructurally and histochemically, a predominant endocrine differentiation was present in the undifferentiated areas of the tumors. These two cases lend further support to the recent concept of a pluripotential stem cell in the mucosa of the gastrointestinal tract capable of differentiation in several directions.