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Multidrug resistance has been a major problem in cancer chemotherapy. In this study, in vitro and in vivo modulations of MDR by ginsenoside Rg3, a red ginseng saponin, were investigated. In flow cytometric analysis using rhodamine 123 as an artificial substrate, Rg3 promoted accumulation of rhodamine 123 in drug-resistant KBV20C cells in a dose-dependent manner, but it had no effect on parental KB cells. Additionally Rg3 inhibited [3H]vinblastine efflux and reversed MDR to doxorubicin, COL, VCR, and VP-16 in KBV20C cells. Reverse transcriptase-polymerase chain reaction and immuno-blot analysis after exposure of KBV20C cells to Rg3 showed that inhibition of drug efflux by Rg3 was due to neither repression of MDR1 gene expression nor Pgp level. Photo-affinity labeling study with [3H]azidopine, however, revealed that Rg3 competed with [3H]azidopine for binding to the Pgp demonstrating that Rg3 competed with anticancer drug for binding to Pgp thereby blocking drug efflux. Furthermore, Rg3 increased life span in mice implanted with DOX-resistant murine leukemia P388 cells in vivo and inhibited body weight increase significantly.