Abstract: In the present study, the protein expression of cyclin D1 was determined in human breast cancer T47D cells and its TAM-resistant subline (T47D/TAMR-6 cells). The resistant subline was established in vitro following stepwise increase in TAM concentration in the culture medium of parent cells. Immunocytochemical method was used to determine the expression of cyclin D1 in both parent and resistant cells. The slow growing T47D/TAMR-6 cells showed lower level of expression of cyclin D1 protein compare to parent T47D cells. Expression of cyclin D1 protein was significantly decreased in both cell types following exposure to TAM (1 μM). Therefore, it can be suggested that TAM can regulate the expression of cyclin D1 protein that affects the cell proliferation. In conclusion, determination of protein expression of cyclin D1 in tumor samples of patients would be helpful in tumor prognosis and prediction of outcome of chemo-hormonal therapy of breast cancer.