The human ovarian cancer cell lines A2780 and A2780/CP70 were studied to investigate the cellular basis for their relative sensitivities to tetrachloro(DL-trans)-1,2-diamminecyclohexaneplatinum(IV) (ormaplatin). Cells were exposed to ormaplatin for 1 h in all experiments. As assessed by colony formation assays, the A2780/CP70 cell line [50% inhibitory dose (IC50) = 3.6 microM] was 9.5-fold more resistant to ormaplatin than the A2780 cell line [IC50 = 0.38 microM]. For cisplatin, the IC50 doses were 40 and 3 microM, respectively. Both cell lines were treated with ormaplatin at doses ranging from 0.10 to 40 microM, for the purpose of studying drug accumulation and efflux, and DNA adduct formation and repair. When these cell lines were treated at their respective IC50 doses, drug accumulation was greater in the resistant cells. When treated at equal microM doses, the sensitive cells formed 8-fold more DNA adduct than the resistant cells. When cells were treated with ormaplatin so as to achieve equivalent levels of platinum-DNA modification, sensitive cells removed 53% of the platinum-DNA damage in the first 6 h after drug exposure, compared to 68% in the resistant cells. We conclude that in human ovarian cancer cells made resistant to cisplatin, there is moderate cross-resistance to ormaplatin. This cross-resistance is not explained by differences in drug accumulation but is associated with reduced platinum-DNA adduct formation, which may be attributable in part to cytosolic inactivation of drug.