Quercetin (3,3',4',5,7-pentahydroxyflavone) has been shown to inhibit a variety of enzymes including the calcium- and phospholipid-dependent protein kinase (protein kinase C) in vivo and in vitro. We show that this compound synergistically enhances the antiproliferative activity of cis-diamminedichloroplatinum(II) (cis-DDP) and nitrogen mustard. Quercetin does not affect the repair of DNA interstrand cross-links introduced by cis-DDP. Long-term exposure to 12-O-tetradecanoylphorbol-13-acetate (TPA), which reduces total protein kinase C activity, also amplifies the growth-inhibitory effect of cis-DDP and acts synergistically with quercetin. A synergism is also observed if tamoxifen or staurosporine are combined with cis-DDP. For both drugs the dose-effect curves for the inhibition of protein kinase C closely resemble the dose-effect curves for the antiproliferative activities. Although alternative mechanisms cannot be definitively excluded, the effects of quercetin, TPA, tamoxifen and staurosporine may result from the inhibition of protein kinase C.