The BCL-2 family has various pairs of antagonist and agonist proteins that regulate apoptosis. Whether their function is interdependent is uncertain. Using a genetic approach to address this question, we utilized gain- and loss-of-function models of Bcl-2 and Bax and found that apoptosis and thymic hypoplasia characteristic of Bcl-2-deficient mice are largely absent in mice also deficient in Bax. A single copy of Bax promoted apoptosis in the absence of Bcl-2. In contrast, overexpression of Bcl-2 still repressed apoptosis in the absence of Bax. While an in vivo competition exists between Bax and Bcl-2, each is able to regulate apoptosis independently.