We studied the effects of tannic acid and 1(beta),2,3,4,6-penta-O-galloyl-D-glucose (pentagalloylglucose), one of the components of tannic acid, on the P-glycoprotein (P-gp) function in multidrug-resistant P-gp over-expressing KB-C2 cells. Both tannic acid and pentagalloylglucose markedly elevated the accumulation of P-gp substrates, rhodamine 123 and daunorubicin, by inhibiting their efflux. A 19-fold increase in cellular rhodamine 123 was observed for tannic acid at 60 microM (85 microg mL(-1)) and a 21-fold increase was observed for pentagalloylglucose at 100 microM (94 microg mL(-1)). The increasing effects of these compounds in the accumulation were much larger than that of (-)epigallocatechin-3-O-gallate (EGCG), which has been revealed to have a prominent inhibitory effect on P-gp compared with other flavonoids. Analysis of verapamil-stimulated ATPase activity in membrane vesicles expressing human P-gp suggested that inhibition of P-gp function by tannic acid and pentagalloylglucose was at least partly due to ATPase inhibition of P-gp. The findings also suggested that the presence of a large number of galloyl groups in polyphenols strengthens the interaction with regulatory regions in P-gp.