Abstract Lymphoid tumors often respond poorly to conventional cytotoxics, a common cause being their impaired sensitivity to apoptosis, such as that caused by Bcl-2 overexpression. A strategy to overcoming this is to use mimics of the natural antagonists of pro-survival Bcl-2, the BH3 only proteins. A promising BH3 mimetic is ABT-737, which targets Bcl-2 and closely related pro-survival proteins. We evaluated its potential utility by testing it on cell lines, clinical samples and on a relevant mouse lymphoma model. We assessed the sensitivity of B cell lymphoma cell lines and primary CLL samples to ABT-737, either alone or in combination. To ascertain its efficacy in vivo, we utilized a mouse model based on the Eμ-myc tumor that is readily transplantable and amenable to genetic manipulation. When syngeneic recipient mice were inoculated with tumors, they develop widespread lymphoma, fatal unless treated by agents such as cyclophosphamide. We found that ABT-737, on its own, was cytotoxic only to a subset of cell lines and primary CLL samples. However, it can synergize potently with agents such as dexamethasone, suggesting that this agent might be useful in combination with currently used chemotherapeutics. In the Eμ myc mouse lymphoma model, treatment with ABT-737 alone did not control the disease as multiple independently derived tumors proved refractory to treatment with this agent. However, ABT-737 was partially effective as a single agent for treating bitransgenic tumors derived from crosses of the Eμmyc and Eμ-bcl-2 transgenic mice. ABT-737 therapy prolonged the survival of recipient mice transplanted with tumors from 30 to 60 days. When combined with a low dose of cyclophosphamide (50mg/kg), long term stable remissions were achieved, which were sustained even longer than control mice treated with much higher doses of cyclophosphamide (300mg/kg). We found that ABT-737 was well tolerated as a single agent and when combined with low doses of cytotoxics such as cyclophosphamide. Thus, ABT-737 may prove to be efficacious for those tumors highly dependent on Bcl-2 for their survival. We found that despite its high affinity for Bcl-2, Bcl-xL and Bcl-w, many cell types proved refractory to ABT-737 as a single agent. We show that this resistance reflects its inability to target another pro-survival relative Mcl-1. Down-regulation of Mcl-1 by several strategies conferred sensitivity to ABT-737. Furthermore, enforced Mcl-1 expression in the Eμmyc/bcl-2 bitransgenic mouse lymphoma model conferred marked resistance as mice transplanted with such tumors died as rapidly as the untreated counterparts. However, enhanced Bcl-2 overexpression on these tumors had little impact on the in vivo response, suggesting that ABT-737 can be utilized even when Bcl-2 is markedly overexpressed. ABT-737 appears to be a promising agent for the clinic. It potently sensitizes certain lymphoid tumors to conventional cytotxics in vitro. The synergy observed between dexamethasone and ABT-737 on some lymphoid lines in culture suggests that it is attractive for clinical testing. Encouragingly, ABT-737 appeared efficacious in vivo against Bcl-2 overexpressing tumors when combined with a reduced dose of cyclophosphamide, suggesting that it will be useful for treating even those Bcl-2-overexpressing tumors that are normally highly chemoresistant.