The antitumor activities of five anticancer drugs, at IC50 dosages cisplatin (CDDP), adriamycin (ADM), etoposide (VP-16), mitomycin C (MMC) and carboplatin (CBDCA) were studies an 7,12-dimethylbenz (a) anthracene induced rat ovarian cancer cell line (DMBA-OC-1) and a human ovarian serous adenocarcinoma cell line (KOC-1S). The IC50 dosage of anticancer drugs for DMBA-OC-1 was: CDDP 0.2 microgram/ml. ADM 0.04 microgram/ml, VP-16 3.0 microgram/ml, MMC 0.1 microgram/ml and CBDCA 10.0 micrograms/ml. The results of our study except those for MMC were parallel with those published on in vivo studies. The IC50 dosages of DMBA-OC-1 did not have enough antitumor activity for KOC-1S, whereas the original KOC-1S tumor strongly resisted the combination chemotherapy (CDDP, ADM and cyclophosphamide). Therefore, our findings suggested the possibility of the separation of multiple drug resistant clones from KOC-1S. These two cell lines had quite different antitumor activity characteristics.