In order to determine the clinical significance of sialyl SSEA-1 antigen, we compared its usefulness as a tumor marker for ovarian cancer with simultaneously measured CA125, CA19-9, TPA, IAP, CEA and ferritin. The sialyl SSEA-1 antigen in serum was measured by radioimmunoassay with an "FH-6" Otsuka Kit. The immunohistochemical localization of sialyl SSEA-1 antigen in ovarian carcinoma tissues was determined by an immunoperoxidase method using FH-6 monoclonal antibody. Among fifty-one patients with ovarian cancer, the incidence of elevated serum levels was 54.9% with sialyl SSEA-1 antigen, 90.2% with CA125, 48.8% with CA19-9, 78.0% with TPA, 73.1% with IAP, 17.1% with CEA and 63.4% with ferritin. On the other hand, among the patients with uterine malignancies and gynecologic benign tumors, the incidence of elevated sialyl SSEA-1 antigen levels in serum was lower than that of other tumour markers. In the patients with ovarian cancer, the serum levels of sialyl SSEA-1 antigen increased in accordance with the advance of the clinical stage and were also correlated with the effect of therapy. In the examination of immunohistochemical localization of sialyl SSEA-1 antigen, a positive reaction occurred in 10 out of 30 ovarian carcinoma specimens. Intense staining appeared in the secretory materials, in the luminal surface of the glands, and in the cytoplasm of cells. Thus, sialyl SSEA-1 antigen appears to be a useful tumor marker for the diagnosis of ovarian cancer, especially when measured simultaneously with CA125, CA19-9, TPA, ferritin and IAP.