Objective: To investigate the sensitization of physiological (10~(-9)mol/L) and pharmacological (10~(-5)mol/L) concentrations of melatonin on cell line MCF-7 for adriamycin and its mechanism. Methods: (1) MTT was applied to test the changes in inhibition ratio and IC_(50) of call line MCF-7 for adriamycin before and af-ter incubation with melatonin. (2) Flow cytometry was used to observe the effect of different concentrations of melatonin, adriamycin and melatonin plus adriamycin on cell apoptosis. (3) Western blot was employed to de-termine the expression of p53 and bcl-2 in MCF-7 cells incubated with melatonin, adriamycin and melatonin plus adriamycin. Results: (1) MTT method showed that adriamycin had inhibitive effect on the growth of MCF-7 cells in a dose- and time-dependent manner. The IC_(50) of cell line MCF-7 for adnamycin before treat-ment with melatonin was 0.62±0.07ug/mL (P>0.05). The IC50 of cell line MCF-7 for adriamycin incubated with physiological and pharmacological concentrations of melatonin was 0.59±0.09ug/mL and 0.42±0.02ug/mL, re-spectively, with a significant difference (P<0.01). (2) Flow cytometry method showed that adriamycin could promote apoptosis of MCF-7, and no changes in the apoptosis index were observed as the concentration of melatonin was changed (P>0.05). With the same concentration of adriamycin, the apoptosis index of cells treated with physiological concentration of melatonin plus adriamycin was not changed (P=>0.05), but the apop-tosis index of cells treated with pharmacological concentrations of melatonin plus addamycin was increased significantly. The concentration of adriamycin had no effect on the apoptosis index. (3) Western blot showed that P53 protein was expressed at a lower level and bcl-2 protein was highly expressed. Physiological concen-trations of melatonin increased the expression of p53 and decreased bcl-2 expression in a dose - dependent manner. The concentration of addamycin had no effect on the expression of p53 and bcl-2 proteins. Conclu-sion: (1) Physiological concentrations of melatonin had no effect on the anti-cancer effect of adriamycin. Phar-macological concentrations of melatonin showed sensitization of MCF-7 cells for adriamycin. (2) With a lower concentration of adriamycin, the promotion of apoptosis may be part of the mechanism of sensitization effect of melatonin. With the increase of adriamycin concentration, the cytotoxic mechanism of melotonin became more and more important. (3) Physiological concentration could increase the expression of p53 and decrease bcl-2 expression in ER~+ breast carcinoma cell line MCF-7 in a dose -dependent manner. The apoptosis involv-ing p53 and bcl-2 passway was part of the mechanism of sensitization effect of melatonin for addamycin.