Bak has been shown to both promote apoptosis and to inhibit cell death while two other members of the Bcl-2 family of proteins, Bcl-XL and Bcl-2 delay apoptosis induced by various stimuli including chemotherapeutic agents. We generated clones with stable expression of Bak wild-type (wt) and Bak with its BH3 (delta78-86) domain deleted (deltaBH3) in FL5.12 cells or FL5.12 cells expressing either Bcl-XL or Bcl-2 to determine if Bak could accelerate apoptosis and antagonize the death repressor activity of Bcl-XL and Bcl-2 during chemotherapy-induced apoptosis. We found that Bak accelerated cell death in FL5.12 cells treated with etoposide, fluorouracil or taxol. In FL5.12 cells expressing Bcl-XL and Bak wt or Bak deltaBH3, both Bak wt or Bak deltaBH3 were able to antagonize the protective effect of Bcl-XL when treated with etoposide or fluorouracil. Bak wt or Bak deltaBH3 were also able to abrogate the protective effect of Bcl-2 in cells expressing Bcl-2 and Bak wt or Bak deltaBH3 when challenged by etoposide or fluorouracil. Immunoprecipitation studies revealed that deletion of BH3 disrupted heterodimerization between Bak and Bcl-XL and that both Bak wt and Bak deltaBH3 failed to interact with Bcl-2. These results demonstrate that Bak does not require its BH3 domain to promote apoptosis in stably transfected cells. Furthermore, Bak can accelerate chemotherapy-induced cell death independently of its heterodimerization with Bcl-XL and Bcl-2.