We have previously shown that the toxicity of valinomycin (VM), a membrane-active agent with antineoplastic activity, can be dramatically reduced with no loss of the antitumor efficacy of the drug by incorporating it into liposomes. In the present study, we investigated the interaction between cis-diamminedichloroplatinum(II) (CDDP) and VM in terms of in vitro cytotoxicity to human ovarian tumor cells. Using the MTT assay and analyzing the data using the median-effect principle, we showed that synergistic cytotoxic interactions exist between CDDP and VM in their liposomal form. The degree of cytotoxic synergism was influenced by the duration of drug exposure and the dose ratio. The cellular accumulation of platinum by ovarian cells at 37 degrees C was slightly higher after exposure to VM as compared with controls; however, it is not clear that this accounts for the cytotoxic synergism. These results suggest that the combination of liposomal VM and CDDP may have merit as a form of localized drug delivery for the treatment of ovarian cancer disseminated within the peritoneal space.