Abstract: Objective To discuss the effect of soybean isoflavones on initiation and progression of breast cancer in MMTV-erbB-2 transgenic female mice.Methods One hundred MMTV-erbB-2 transgenic female mice were divided into four groups:early intervention,advanced intervention,full intervention,and control group.The incidence,latent period,tumor location,and tumor growth rate of breast cancer in the mice were observed.Results The incidence of breast cancer in early intervention,advanced intervention,full intervention,and the control group were 48%,28%,20%,and 92%,respectively. Compared with the control group,the incidence rate of other three groups was decreased significantly(P < 0.01).The inci dence rate of full intervention group was lower than that of early in tervention group(P < 0.05).There was no difference in the incidence rate between the early intervention group and advanced intervention group and full intervention group(P> 0.05).There was no significant difference in the latentperiod of breast cancer(P > 0.05).Tumor location in the four groups was at the three pairs of mammary gland.And the rate of tumor growth showed no significant difference in four groups (P > 0.05).Conclusion Soybean isoflavones have preventive effect on the initiation and development of breast cancer in MMTV-erbB-2 transgenicmice.