Breast cancer is the second most commonly diagnosed cancer in women worldwide. MCF-7 cell line is an extensively studied human breast cancer cell line. This cell line expresses estrogen receptors, and the growth of MCF-7 cells is hormone dependent. In this study, I propose a mathematical model governing MCF-7 cell growth with interaction among tumor cells, estradiol, natural killer (NK) cells, and white blood cells (WBCs). Experimental data are used to determine functional forms and parameter values. Breast tumor growth is then studied using the mathematical model. The system has three coexisting stable equilibria representing the tumor free state, a microscopic tumor, and a large tumor. The results obtained from numerical simulation are compared with those from clinical and experimental studies.