Abstract: Objective To explore the effect of daidzein on the apoptosis of H22 tumor cells and its mechanism in mice. Methods The proliferation of H22 cells was detected by methyl thiazlyl tetrazolimm(MTT)assay.Sixty Kunming male mice were randomly divided into 6 groups(10 in each group).The mice in model and normal control group were given intraperitoneal injection of serum free RPMI 1640.The high,moderate,and low dose of daidzein groups were injected intraperitoneally with daidzein at different dose(50,100,and 200 mg/kg).The mice in positive control group were injected intraperitoneally with cyclophosphamide solution.After 14 days,the body weight,spleen index,thymus index,and tumor weight of the mice in different groups were compared.Bax and bcl-2 expressions in H22 cells were detected with western-blot. Results MTT assays showed that daidzein could inhibit the proliferation of H22 cells.After exposed to daidzein at a dose of 150 μmol/L for 72 hr,the proliferation inhibitory rate of the H22 cells was 30.3% (P<0.05).Compared to that of previous culture cycle,the growth inhibitory rate gradually increased.Thymus and spleen indexes in high and moderate dose daidzein groups were significantly higher(P<0.01).Compared with that of the model group,the tumor weight index of the three daidzein treatment groups decreased significantly(P<0.01 for all).Daidzein could regulated upward the expression of bcl-2 and downward the expression of bax.Compared with the 10.0μmol/L daidzein treatment group,the gray value for bax expression increased by 50.05% and that of bcl-2 expression decreased by 52.62%. Conclusion Daidzein inhibits the proliferation of H22 cells and induces apoptosis of H22 tumor cells.Daidzein can inhibit the growth of implanted tumor and promote the immune function of the mice.