Human granulocyte-macrophage colony-stimulating factor (GM-CSF) is a regulatory glycoprotein that stimulates the production of granulocytes and macrophages from committed hematopoietic progenitor cells both in vitro and in vivo. In this report, we show that recombinant human GM-CSF enhances colony formation by nonhematopoietic human ovarian cancer cell lines, IGROV-1, A2774, ME-180, Pa-1 and A2780. GM-CSF also enhanced the colony formation by cells obtained from fresh ascites of a patient with ovarian mucinous cystadenocarcinoma and a patient with serous papillary ovarian carcinoma. Our observations were made with GM-CSF concentrations between 0.1 to 1 ng/ml; these concentrations are equivalent to the dosages generally used for bone marrow recovery after chemotherapy.