Using a Bax-dependent membrane-permeabilization assay, we show that peptides corresponding to the BH3 domains of Bcl-2 family "BH3-only" proteins have dual functions. Several BH3 peptides relieved the inhibition of Bax caused by the antiapoptotic Bcl-x_L and/or Mcl-1 proteins, some displaying a specificity for either Bcl-x_L or Mcl-1. Besides having this derepression function, the Bid and Bim peptides activated Bax directly and were the only BH3 peptides tested that could potently induce cytochrome c release from mitochondria in cultured cells. Furthermore, Bax activator molecules (cleaved Bid protein and the Bim BH3 peptide) synergistically induced cytochrome c release when introduced into cells along with derepressor BH3 peptides. These observations support a unified model of BH3 domain function, encompassing both positive and negative regulation of other Bcl-2 family members. In this model, the simple inhibition of antiapoptotic functions is insufficient to induce apoptosis unless a direct activator of Bax or Bak is present.