We compared antisense phosphorothioate oligonucleotides (PS-ODN) that target BCL-2 such as Genasense (G3139-PS), with other PS-ODN or phosphodiester-ODN (PO-ODN) in their relative capacity to induce apoptosis of chronic lymphocytic leukemia (CLL) B cells in vitro. Surprisingly, we found that thymidine-containing PS-ODN, but not PO-ODN, induced activation and apoptosis of CLL cells independent of BCL-2 antisense sequence or CpG motifs. All tested thimidine-containing PS-ODN, irrespective of their primary sequences, reduced the expression of Bcl-2 protein and increased the levels of the proapoptotic molecules p53, Bid, Bax in CLL cells. Apoptosis induced by thymidine-containing PS-ODN was preceded by cellular activation, could be blocked by the tyrosine-kinase inhibitor imatinib mesylate (Gleevec), and was dependent on ABL kinase. We conclude that thymidine-containing PS-ODN can activate CLL cells and induce apoptosis via a mechanism that is independent of BCL-2 gene interference or CpG motifs.