It has been demonstrated that the flavonoid quercetin (3,3',4',5–7-pentahydroxyflavone) (Q) inhibits the growth of several cancer cell lines and that the antiproliferative activity of this substance is mediated by a so-called type II estrogen binding site (type II EBS). We investigated the effects of quercetin and cisplatin (CDDP) alone and in combination on the proliferation of the ovarian cancer cell line OVCA 433. Both drugs exhibited a dose-related growth inhibition in a range of concentrations between 0.01 and 2.5 μM and 0.01 and 2.5 μg/ml for Q and CDDP respectively. The combination of the two drugs resulted in a synergistic antiproliferative activity. Two other flavonoids tested, i.e., rutin (3-rhamnosylglucoside of quercetin) and hesperidin [7-b rutinoside of hesperetin (3'-5–3-hydroxy-4-methoxyflavone)] were ineffective both alone and in combination with CDDP. Since both rutin and hesperidin do not bind to type II EBS it can be hypothesized that Q synergizes with CDDP by acting through an interaction with these binding sites.