The mitogenic effect of 17β-estradiol (E2) on the breast is mediated by estrogen receptor alfa (ERα), hence ERα antagonists are effective in the treatment of breast cancer. The possible use of estrogen receptor beta (ERβ) as a target in treatment of breast cancer is under investigation. The mouse mammary cell line HC11 expresses both ERs and was used to study the role of the two receptors in proliferation. E2 had no effect on proliferation. The ERα-selective agonist 4, 4′, 4 ″-(4-propyl-(1H)-pyrazole-1, 3, 5-triyl) trisphenol (PPT) stimulated proliferation. The ERβ-selective agonist 2, 3-bis (4-hydroxy-phenyl)-propionitrile (DPN) inhibited cell growth and induced apoptosis. PPT upregulated while DPN downregulated cyclin D1 and proliferating cell nuclear antigen (PCNA). Upon inhibition of ERα expression with RNA interference, E2 caused a decrease in cyclin D1 and PCNA, and increased apoptosis. When ERβ expression was blocked, E2 induced proliferation and cells gained the capacity to grow in soft agar. In summary, in HC11 mammary epithelial cells, ERα drives proliferation in response to E2 while ERβ is growth inhibitory. The lack of effect of E2 on HC11 cell growth is the result of the combined actions of ERα (proliferation) and ERβ (apoptosis). We suggest that use of ERβ agonists will be a useful addition in treatment of breast cancer, which, at present, is only aimed at inhibition of ERα.