Placental protein 5 (PP5), a serine protease inhibitor, was measured by radioimmunoassay in fluids aspirated from benign and malignant cystic ovarian tumours. PP5 was found in 7 out of 9 malignant and in 12 out of 29 benign cyst fluids (p less than 0.01), the levels ranging from 3.0 to 73.4 micrograms/l. No PP5 was found in the corresponding serum samples from 4 patients whose malignant cystadenocarcinomas contained PP5. In those tumours where PP5 was found, no difference was observed in the PP5 levels between benign and malignant, or between serous and mucinous tumours.