Bak and Bax, pro-apoptotic members of the Bcl-2 family mediate the mitochondrial pathway of apoptosis. Following an apoptotic stimulus, the binding of BH3-only relatives at a hydrophobic surface groove (α2-α5) triggers Bak (and Bax) activation. Activation involves a series of Bak conformation changes resulting in the formation of symmetric homodimers, which then associate to form the apoptotic pore in the mitochondrial outer membrane. We report that antibodies can directly trigger activation of Bak and of mitochondrial Bax, and that they do so by binding to the α1-α2 loop. The mechanism of antibody-mediated Bak activation involved displacement of the α1 helix, revealed by biochemical studies and a structural model of Fab bound to Bak. Downstream conformation changes in Bak were similar to those triggered by BH3-only proteins binding at the α2-α5 groove. Intriguingly, antibodies to the α1-α2 loop in cytosolic Bax blocked translocation triggered by either Bid. Thus, our results identify the α1-α2 loop as a new target for regulating Bak and Bax apoptotic function, and highlight the use of antibodies as valuable tools to study Bak and Bax activation.