To clarify the significance of the Fas antigen (Ag) in gynecologic tumors, its expression in gynecologic cancer cell lines was examined. The Fas Ag was expressed in 6 of 15 cell lines. Five of 8 ovarian cancer cell lines but none of 4 choriocarcinoma cell lines expressed the Fas Ag. In drug-resistant cell lines derived from one of the Fas-positive cells, its expression was not lost after development of resistance to cisplatin, SN-38 or etoposide, but its expression was absent in the cell line resistant to Adriamycin. The effect of the anti-Fas antibody (Ab) was then studied. Apoptosis was induced in 7 of 9 Fas-positive cell lines, whereas the remaining two cell lines were unaffected. Furthermore, the combination effect of the anti-Fas Ab and drugs was examined in an ovarian cancer cell line and its drug-resistant variants, and a synergistic effect was observed. These results suggest important roles of the Fas Ag in ovarian cancer and the potential for overcoming drug resistance by a combination of the anti-Fas Ab and various drugs.