The cellular components of ascitic fluid aspirated from the peritoneal cavity of women with advanced ovarian cancer were separated on a Ficoll gradient. Isolated macrophages, which were further purified, elaborated a growth factor which was mitogenic for human endothelial cells isolated from umbilical veins, arteries and the omental microvasculature in vitro, and was angiogenic in vivo. It is postulated that the macrophage-derived factor enhances tumor neovascularization of the widespread ovarian-derived peritoneal malignant lesions appearing in these patients, thus contributing to their rapid growth and metastasis, and the poor prognosis of the disease.