By continuous exposure to either cis-diamminedichloroplatinum (II) (CDDP) or adramycin (ADM), NOS2 cells, derived from human serous cystadenocarcinoma of the ovary, acquired 7 times resistance to CDDP and 24 times stronger resistance to ADM, respectively. The ADM-resistant cells showed cross-resistance to more drugs than the CDDP-resistant cells. We examined the usefulness of combination therapy with CDDP and ADM in eight cell lines derived from human ovarian carcinomas and CDDP - as well as ADM - resistant cell lines using either isobologram or combination index (CI); both examinations revealed additive effects in seven of the eight cell lines and antagonistic effects in one cell line. Furthermore, we studied effects of CDDP combined with 5-fluorouracil (5-FU), etoposide (VP16) or camptothecin (CPT11) by CI method, which had a lower value in the regimen combined with 5-FU than that with ADM, representing a favorable effect. VP16 and CPT11 had almost the same CI value as thatn of ADM. There is no evidence that the combination of ADM and CDDP is more useful than that of any other anticancer drug and CDDP in vitro.