The purpose of the present study is to investigate the cytotoxicity/apoptotic effect of 2-chloro-2′-deoxyadenosine, cladribine, (2-CdA) in the human breast cancer cell line, MDA-MB468 (estrogen receptor negative, ER−). MTT [3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H tetrazolium bromide] assay, annexin V-Fluorescein/PI and Hoechst 33258 staining were used to detect cytotoxicity and cell apoptosis. The activation of caspase-3 and -9 was assayed using caspase activation assay kits. Gel electrophoresis was performed to detect DNA fragmentation. Treatment of MDA-MB468 cells with different concentrations of 2-CdA (50, 100 and 500 µM) resulted in a significant increase in the cell death. Annexin V-Fluorescein/PI and Hoechst 33258 staining revealed that the cell death was mainly of apoptotic type. DNA laddering profile was also obtained in the treated MDA-MB468 cells using DNA fragmentation analysis. A significant (p<0.05) increase in the activity of caspase-3 and -9 was observed. Pre-treatment of the cells with kinase inhibitor, 5′-amino-5′-deoxyadenosine inhibited the cytotoxicity effect of 2-CdA. This suggests that intracellular phosphorylation activation reaction plays a key role in the 2-CdA-induced apoptosis. In conclusion, this study showed that high dose of cladribine has an apoptotic effect on ER−MDA-MB468 breast cancer cells and its intracellular phosphorylation is necessary for cytotoxicity.