The effects of estradiol and tamoxifen on the proliferation of estrogen receptor positivecells and the relationship between the tamoxifen tolerance and cell origin were investigated. The tis-sues of human endometrium and breast cancer were randomly selected following dissection for pri-mary cell culture. After the breast cancer cells and endometrial cells were treated with 1 × 10-8 mol/L estradiol and/or 1 × 10-6 tamoxifen, a H-labelled thymine nucleotide was used to trace the kineticsof cell proliferation. There was no significant difference in the inhibition on the human endometrialcells between tamoxifen-treated group (6.3%) and control group (6.4%), but tamoxifen could sig-nificantly inhibit the proliferation of the human breast cancer cells (45.84 % ) as compared with con-trol group (52.72%). Moreover, tamoxifen could significantly stimulate the proliferation of tamox-ifen resistant breast cancer cells (9.64%) as compared with control group (6.32 %). Estradiolcould significantly stimulate the proliferation of all the three kinds of cells as compared with controlgroup. The combined use of estradiol and tamoxifen could inhibit the proliferation of the endometri-al cells and breast cancer cells as compared with estradiol used alone, but on the tamoxifen resistantbreast cancer cells, they could more significantly stimulate the proliferation than E2. It was conclu-ded that E2 could stimulate the proliferation of these three kinds of cells. However, the inhibitiveeffects of tamoxifen on the proliferation of these cells were dependent on the estradiol.