ABSTRACT M11L, a 166-amino-acid antiapoptotic protein of myxoma virus, was previously shown to bind to the peripheral benzodiazepine receptor by hydrophobic interactions at the outer mitochondrial membrane. Here we demonstrate that an additional property of M11L is the ability to constitutively form inhibitory complexes with the proapoptotic Bcl-2 family member Bak in human cells. This binding interaction was identified by both FLAG-tagged pull-down assays and tandem affinity purification from transfected and virus-infected human cells. M11L binds constitutively to human Bak and, under some inducible conditions, to human Bax as well, but not to the other Bcl-2 family members (Bad, Bid, Bcl-2). When stably expressed in human embryonic kidney (HEK293) cells, M11L effectively protects these cells from Fas ligand-induced apoptosis, thereby blocking release of cytochrome c , activation of caspase 9, and cleavage of poly(ADP-ribose) polymerase. We also demonstrate in coexpression studies that M11L can interact with Bak independently of any involvement with Bax. Furthermore, cells stably expressing M11L function to prevent apoptosis that is induced by overexpression of Bak. We conclude that M11L inhibits, in a species-independent fashion, apoptotic signals mediated by activation of Bak.