The expression of Thomsen-Friedenreich antigen (T-Ag), the precursor of MN blood group antigen, was studied by the ABC method with Arachys Hypogaea lectin (PNA) and the relationship of the results to the histology and clinical behavior of primary gynecologic cancers were examined. The results were as follows: 1. In cervical squamous cell tumors, the incidence of the expression of T-Ag in invasive cancers (52.9%, 37/70) was significantly higher (p less than 0.05) than that in intraepithelial tumors (28.6%, 8/28). No cryptic T-Ag(-) tissue was found in intraepithelial tumors or microinvasive cancers. 2. In squamous cell carcinomas of the cervix, the incidence of cryptic T-Ag(-) tissue was high (36.4%, 4/11) in tissue of the small cell non-keratinizing type, the most undifferentiated type, whereas it was not found in 5 cases of well-differentiated keratinizing carcinoma examined. 3. In stage Ib and II cervical cancers, no relationship was recognized between the expression of T-Ag in primary lesion and extension to the parametrium or the pelvic lymph nodes. 4. The incidence of T-Ag(+) tissue in adenocarcinomas of the uterine cervix (62.5%, 5/8), the endometrium (91.7%, 11/12), and the ovarian (80%, 8/10) was higher than that in squamous cell carcinoma of the uterine cervix (52.5%, 32/61). 5. In endometrial cancers, no relationship was recognized between the expression of T-Ag and hormone receptor status.