Abstract: ObjectiveTo study the antitumor effect of genistein (Genistein GEN) in cultured drug-resistant breast cancer cell line of MCF-7/ADM in vitro, and influences of genistein to cell cycle and apoptosis. MethodsInhibitory effect of GEN alone or combined with doxorubicin on the cultured MCF-7/ADM was detected by MTT assay; the accumulative effect of GEN on doxorubicin in MCF-7/ADM was detected by fluorescence spectrophotometry; and cell cycle and apoptosis rate was detecced by flow cytometry (FCM). ResultsSignificant inhibitory effect on cultured MCF-7/ADM in vitro was not observed under GEN alone or combined with Doxorubicin 48 h later GEN treated alone, the inhibition increased gradually in time-dependent model. When the concentration of GEN reached 60 μg/ml, inhibition effect was markedly increased (P<0.01). When Doxorubicin was added, the inhibition rate was significant increased compared with the control group (P<0.01), and the inhibition strengthened with the increasing concentration of GEN, concentration of intracellular doxorubicin was also increased. Compared with the control group, the cell cycle were both blocked at G2/M phase, apoptosis was found to be the highest percentage in the combination group (P<0.01), typical hypodiploid apoptotic peak was detected before the G1 phase. ConclusionGEN alone and combined with Doxorubicin had an inhibitory and additive effect on cultured human breast cancer cell line MCF-7/ADM in vitro, it could increase the intracellular accumulation of Doxorubicin and arrest cell cycle at phase G2/M, as well as in inducing significant apoptosis of MCF-7/ADM cells, which may be one of its molecular mechanisms of the reversal of multidrug resistance.