A human macrophage-colony-stimulating-factor(M-CSF) gene inserted into an expression vector (pRc/CMV-MCSF) was transfected into multidrug-resistant (MDR) human ovarian cancer cells (ADlO) to induce secretion of human M-CSF into the medium. The M-CSF level in the culture medium of the transfected cells reached 100 ng/ml after 7 days, and the ability of the cells to secrete M-CSF was stable for at least 3 months. Transfection of the M-CSF gene did not result in any change in expression of MDR I (P-glycoprotein), proliferation or chemosensitivity of the cells from those of the parent cells. There was also no difference between the transfected and the parent cells in susceptibility to NK cell-or interleukin-2-activated killer-cellmediated cytotoxicity. Human blood monocytes that had been incubated for 4 days in medium with the culture supernatant of MH-AD 10 cells exhibited higher ADCC activity than untreated monocytes against MDR I -positive cancer cells. This effect of the supernatant of AD I0 cells was completely abolished by its treatment with a monoclonal anti-M-CSF antibody (MAb). When transfected human MDR cells were injected into nude mice, an inverse correlation was seen between the ability of the cells to produce M-CSF and their tumorigenicity. Thus, gene modification of MDR cancer cells seems hopeful as a therapeutic method for enhancing anti-MDR I -MAb-dependent macrophagemediated cytotoxicity against human MDR cancer cells.