Breast cancer is one of the most common cancer types and malfunctioning proteins in apoptotic pathways are known to be the main contributors of cancer development. In this study MCF-7 and MDA-MB-231 breast cancer cell lines were treated with a potent chemotherapeutic agent, doxorubicin. IC50 values were calculated to be 8306 and 6602 nM, respectively. Percentage of the cells in cell cycle arrest increased in a dose-dependent manner and was more evident in MDA-MB-231 cells. Apoptotic cell number increased in MCF-7 and Bax/Bcl-2 ratio was measured to be similar to 7 fold higher in cells treated with 200 nM of doxorubicin when compared to the control. Likewise, apoptotic cell rate increased in MDA-MB-231 cells and Bax/Bcl-2 ratio was increased by 2 fold compared to the control. Doxorubicin was also found to suppress Mdr-1 in a concentration-dependent manner and the outcome was more evident in MCF-7 cells. Drug efflux assay results were also shown to be consistent with Mdr-1 gene expression levels in both cell lines. Consequently it was concluded that doxorubicin directly influences apoptotic pathway and multidrug resistance in both MCF-7 and MDA-MB-231 cells even though data all together suggest that the effect is more significant on MDA-MB-231.