Bcl-2 inhibits apoptosis induced by a wide variety of stimuli. In contrast, the Bcl-2 homologue, Bax, antagonizes Bcl-2's death protecting function. Bcl-2 forms protein-protein homodimers with itself and heterodimers with Bax, and previous experiments have shown that point mutations in Bcl-2 can abrogate Bax binding while leaving homodimerization intact. These mutagenesis results can be interpreted to suggest that Bcl-2 has separate binding sites that are responsible for homodimer and heterodimer formation. Results from yeast two-hybrid studies have also suggested that homodimerization and heterodimerization reflect distinct modes of interaction. However, using quantitative plate binding assays, we now show that Bax as well as peptides derived from the BH3 domains of Bax and Bak block both Bcl-2/Bax binding and Bcl-2/Bcl-2 binding. Similar assays demonstrate that Bcl-xL can form both homodimers and heterodimers and that these interactions are also inhibited by Bax and the BH3-derived peptides. These results demonstrate that the same binding motifs are responsible for both homodimerization and heterodimerization of Bcl-2 family members.