The selective cyclooxygenase-2 (COX-2) inhibitor celecoxib constitutes the prototype of pro-apoptotic agents acting through the intrinsic death pathway in a Bcl-2 independent manner. To gain further insight into celecoxib-mediated apoptosis regulation at the level of the mitochondria we tested in how far the crucial pro-apoptotic Bcl-2 proteins Bak and Bax were involved using clones of the Bax-deficient Jurkat T-lymphoma cell model either expressing Bak (Jurkat Bak positive) or being negative for Bak (Jurkat Bak negative), or overexpressing Bcl-2 (Jurkat Bcl-2). Celecoxib induced substantial apoptosis in Jurkat Bak-positive cells. Overexpression of Bcl-2 had only limited protective effects. However, loss of Bak-expression conferred almost complete resistance of Jurkat cells to celecoxib-induced apoptosis. Neither enhanced celecoxib-concentrations nor prolonged incubation times were sufficient to normalize apoptotic rates upon celecoxib-treatment in these Bak/Bax-negative cells. In line with that observation, siRNA-mediated silencing of Bak in the Bak-positive Jurkat cells largely reduced the extent of celecoxib-induced cell death. Interestingly, in celecoxib-sensitive Bak-positive cells, celecoxib-treatment resulted in down-regulation of the anti-apoptotic Bcl-2 protein Mcl-1 which may contribute to celecoxib-mediated activation of Bak-dependent apoptosis. Taken together our data clearly show for the first time the functional relevance of Bak for celecoxib-induced apoptosis in Bax-deficient Jurkat T-lymphoma cells.