The Bcl-2 family of proteins regulates apoptosis, the cell death program triggered by activation of certain proteases (caspases). An attractive model for how Bcl-2 and its closest relatives prevent caspase activation is that they bind to and inactivate an adaptor protein required for procaspase processing. That model has been supported by reports that mammalian prosurvival Bcl-2 relatives bind the adaptor Apaf-1, which activates procaspase-9. However, the in vivo association studies reported here with both overexpressed and endogenous Apaf-1 challenge this notion. Apaf-1 could be immunoprecipitated together with procaspase-9, and the Apaf-1 caspase-recruitment domain was necessary and sufficient for their interaction. Apaf-1 did not bind, however, to any of the six known mammalian prosurvival family members (Bcl-2, Bcl-x(L), Bcl-w, A1, Mcl-1, or Boo), or their viral homologs adenovirus E1B 19K and Epstein-Barr virus BHRF-1. Endogenous Apaf-1 also failed to coimmunoprecipitate with endogenous Bcl-2 or Bcl-x(L), or with two proapoptotic relatives (Bax and Bim). Moreover, apoptotic stimuli did not induce Apaf-1 to bind to these family members. Thus, the prosurvival Bcl-2 homologs do not appear to act by sequestering Apaf-1 and probably instead constrain its activity indirectly.