The circulating levels of placental protein 12 (PP12), a somatomedin-binding protein, were measured by radioimmunoassay in 37 postmenopausal women with ovarian tumors. Elevated levels (above 47 micrograms/l) were observed in 16 of 25 patients (64%) with malignant and 2 of 12 patients (17%) with benign tumors. Elevated levels were more frequent and higher in patients with advanced disease. Within 1 week of surgery, the levels fell in 13 of 17 cases (77%). In addition to clinical interest, these results contribute to our knowledge of carrier proteins of growth factors in patients bearing malignant ovarian neoplasms.