Flavonoids are components of plant foods and of many herbal medicines taken in combination with anticancer drugs. We have examined the potential of flavonoids to affect the accumulation and cytotoxicity of 3 cytotoxic drugs [vinblastine (VLB), daunorubicin (DNR), and colchicine (COL)] that are substrates for the ABC transporter, P-glycoprotein in a vinblastine-resistant T-cell leukemia, CEM/VBL(100), that overexpresses P-glycoprotein. The effects of the flavonoids on accumulation and cytotoxicity of these drugs were different depending on the P-gp substrate used. Most of the 30 flavonoids tested decreased DNR accumulation in the VBL-resistant, but not sensitive, leukemia cells. By contrast, flavonoids that inhibited DNR accumulation enhanced the accumulation of fluorescently labeled vinblastine. None of these flavonoids affected COL accumulation. The effects of the flavonoids on the cytotoxicities of these drugs paralleled their effects on accumulation; the same flavonoids decreased DNR cytotoxicity but increased VLB cytotoxicity and had no effect on COL. Verapamil reversed the accumulation deficit and cytotoxicity of all three P-gp substrates. These effects correlated with the effects of flavonoids on P-gp-ATPase activity. Flavonoids that decreased DNR accumulation stimulated DNR-activated P-gp ATPase, whereas flavonoids that increased fluorescently labeled VLB accumulation inhibited VBL-stimulated P-gp ATPase activity, thereby accounting for the decrease or increase in cancer drug accumulation in resistant cells. We conclude that flavonoids often ingested by cancer patients may have different effects on anticancer drugs and that these findings should be considered in designing future combination treatments for cancer patients.