Histone deacetylase inhibitors (HDACi) are compounds that target the epigenome and cause tumor cell-selective apoptosis. A large number of these agents that have different chemical structures and can target multiple HDACs are being testing in clinical trials and vorinostat is now an approved drug for the treatment of cutaneous T-cell lymphoma. Although these agents are showing promise for the treatment of hematologic malignancies, it is possible that different drugs may have different mechanistic, biological, and therapeutic activities. When comparing an HDACi belonging to the hydroxamic acid class of compounds (vorinostat) with a cyclic tetrapeptide (romidepsin), we showed that these agents regulate the expression of a common set of cellular genes, but certain genes specifically responded to each agent. Using the Eμ-myc mouse model of B-cell lymphoma, we showed previously that overexpression of the prosurvival proteins Bcl-2 and Bcl-X_L inhibited the apoptotic and therapeutic activities of the vorinostat. Herein, we compared and contrasted the apoptotic-inducing activities of the hydroxamic acid oxamflatin with romidepsin. Like vorinostat, oxamflatin was unable to kill lymphomas overexpressing Bcl-2 and Bcl-X_L, indicating that these proteins can generally protect cells against this class of HDACi. In contrast, romidepsin was able to induce apoptosis in lymphomas overexpressing Bcl-2 with delayed kinetics of cell death and could mediate therapeutic responses against these lymphomas. However, romidepsin was inactive when Bcl-X_L was overexpressed. These data provide strong support that HDACi of different chemical classes may have subtle yet potentially important differences in their molecular and biological activities. [Mol Cancer Ther 2008;7(5):1066–79]