We have investigated the combined treatment of components of green tea with adriamycin against M5076 ovarian sarcoma, which exhibits low sensitivity to adriamycin. In M5076 tumor-bearing mice, the injection of adriamycin alone did not inhibit tumor growth, whereas the combination of theanine and adriamycin significantly reduced the tumor weight to 62% of the control level. When combined with theanine, effective antitumor activity of adriamycin was observed without an increase in the dosage. Theanine specifically increased the adriamycin concentration in the tumor by 2.7-fold. In contrast, theanine decreased the adriamycin concentrations in normal tissues. On the other hand, in vitro experiments proved that theanine inhibited the efflux of adriamycin from tumor cells, suggesting a theanine-induced increase in the adriamycin concentration in such tumors in vivo. Furthermore, the oral administration of theanine or green tea similarly enhanced the antitumor activity of adriamycin. In conclusion, the combination of theanine with adriamycin showed antitumor efficacy in spite of the non-effective dose of adriamycin on M5076 ovarian sarcoma. We have found that the modulating action of theanine is useful in clinical cancer chemotherapy.