B-cell chronic lymphocytic leukemia (CLL) is a clonal B cell malignancy of morphologically mature, functionally immature B cells. B-cell CLL cells are known to be resistant to killing by anticancer and other agents. This resistance is associated with alterations in apoptosis and cell cycle regulated genes. In our earlier studies, we have demonstrated that CLL cells have differential expression of genes that are associated with apoptosis and cell cycle regulation, including elevated expression of Bcl-2, DAD-1, Cyclin D3 and cyclin dependent kinase 4 inhibitor. Therefore, in this study, in an attempt to study the role of Cyclin D3 in the resistant behavior of CLL cells, Cyclin D3 was down regulated using antisense oligonucleotide (AS-ODN) in WSU-CLL, a human CLL cell line. The down regulation of Cyclin D3 was confirmed by RT-PCR and flow cytometry techniques. The Cyclin D3 expression down-regulated WSU-CLL cells were then tested for their susceptibility to fludarabine, a chemotherapeutic agent. Our results showed that the Cyclin D3 expression down-regulated WSU-CLL cells were more susceptible to fludarabine mediated killing. Following treatment with fludarabine, there was a significant increase in the number of cells undergoing apoptosis in Cyclin D3 expression down-regulated WSU-CLL cells as determined by Annexin-V assay, cell cycle analysis for DNA content, and cytomorphology. Thus, our results indicate Cyclin D3 down regulation increases the killing of WSU-CLL cells with fludarabine by increasing the number of cells undergoing apoptosis.