Histochemical staining has shown that so-called adenoma malignum (the mucinous type of minimal deviation adenocarcinoma [mucinous MDA]) of the uterine cervix expresses gastric phenotypes. The present ultrastructural study was undertaken to explore the fine structure and phenotypic expression of this tumor, and to make comparisons with normal cervical glands and gastric pyloric mucosa. Post-embedding, double-immunogold staining for gastric gland mucous cell mucin (HIK1083-reactive mucin) and lysozyme revealed localization exclusively to the matrix and to the core of the mucin granules, respectively, both in mucinous MDA and gastric pyloric mucosa. Mucin granules of normal cervical gland cells lacked core structures and showed no immunoreactivity with HIK1083 or lysozyme. Th us, mucinous M DA was confirmed to be a tumor expressing gastric phenotypes ultrastructurally. Both markers should be useful for the identification of tumor cells.