Many of the cancer-associated antigens recently have been identified as mucin antigens. However, there are no detailed studies describing the expression of carbohydrates and core proteins of mucin antigens in ovarian tumors. In this study we examined the expression of carbohydrate antigens, which are associated with the earliest steps in mucin glycosylation (Tn and sialosyl-Tn), and the expression of the mucin core protein antigens associated with the MUC1 gene product (mammary-type apomucin) and the MUC2 gene product (intestinal-type apomucin) in 123 ovarian epithelial (mucinous and serous) tumors. In normal ovarian tissues neither Tn, sialosyl-Tn, nor intestinal-MRP antigens (MUC2 gene product) were expressed, except for positive sialosyl-Tn staining of stromal capillaries, while the MUC1 gene product, DF3 antigen, was expressed in the cell apex of the germinal coelomic epithelium when it had plump, slightly elongated, or pseudostratified nuclei. In the benign adenomas Tn and sialosyl-Tn antigens were detected in a small number of mucinous adenomas and rarely in serous adenomas. In contrast, expression of both Tn and sialosyl-Tn antigens was observed in all the adenocarcinomas and in a considerable number of borderline malignancies. DF3 antigen was expressed in many benign serous tumors but not so frequently in benign mucinous tumors; however, it was frequently expressed in the adenocarcinomas and borderline malignancies of both mucinous and serous types. Intestinal-MRP antigen expression increased with the transition of the mucinous tumors from a benign to malignant state, although it was never detected in the serous tumors. Coexpression of DF3 and intestinal-MRP antigens was seen in borderline malignancies and carcinomas of the mucinous tumors. In conclusion, simultaneous expression of Tn and sialosyl-Tn antigens is a highly effective tumor marker in both mucinous and serous tumors of the ovary. Coexpression of DF3 and intestinal-MRP antigens may indicate the malignant potential of ovarian mucinous tumors.