Mucinous tumors of the ovary are generally considered to have a dual origin in histogenesis. Teratomatous elements, however, have recently been reported as appearing in a process of intestinal metaplasia. Malignant transformation has also been felt to occur through intestinalization. To clarify the matter, the relationships among morphological changes in intestinalization, carcinoembryonic antigen and mucus antigens named IMA and M1, which were isolated from intestinal mucosa and ovarian mucinous cyst fluid respectively, were studied in 4 benign, 8 borderline and 6 malignant mucinous tumors of the ovary. Intestinal type and goblet- like cells were sparse, if any, in benign tumors, but conspicuous in borderline and malignant ones. Carcinoembryonic antigen was located in the glycocalyx of intestinal type cells in the benign tumors, but distributed over the entire cytoplasm increasingly with malignancy. These findings lend support to the theory that malignant transformation may occur through intestinal metaplasia in mucous tumors. M1 was positive in many cells of all mucinous tumors, but IMA was shown in some goblet-like cells of a few tumors, regardless of malignancy or the presence of argyrophil cells. These phenomena could be best understood by postulating that IMA was located in the teratomatous intestinal goblet cells of the tumors.