Celecoxib, a selective cyclooxygenase-2 (COX-2) inhibitor, is the only non-steroidal anti-inflammatory drug so far which has been approved by the FDA for adjuvant treatment of patients with familial adenomatous polyposis. The molecular mechanism responsible for the anti-cancer effects of celecoxib is not fully understood. There is little data on the potential role of COX-2 in lymphoma pathogenesis. In view of the reported induction of apoptosis in cancer cells by cyclooxygenase-2 inhibitors, the present study is undertaken to test the effect of celecoxib on human chronic myeloid leukemia cell line, K562 and other hematopoietic cancer cell lines like Jurkat (human T lymphocytes), HL60 (human promyelocytic leukemia) and U937 (human macrophage). Treatment of these cells with celecoxib (10-100 microM) dose-dependently, reduced cell growth with arrest of the cell cycle at G0/G1 phase and induction of apoptosis. Further mechanism of apoptosis induction was elucidated in detail in K562 cell line. Apoptosis was mediated by release of cytochrome c into the cytoplasm and cleavage of poly (ADP-ribose) polymerase-1 (PARP-1). This was followed by DNA fragmentation. The level of anti-apoptotic protein Bcl-2 was decreased without any change in the pro-apoptotic Bax. Celecoxib also inhibited NF-kB activation. Celecoxib thus potentiates apoptosis as shown by MTT assay, cytochrome c leakage, PARP cleavage, DNA fragmentation, Bcl-2 downregulation and possibly by inhibiting NF-kB activation.