bax is an apoptosis-inducing member of the bcl-2 multigene family. We have studied interactions of human Bax with itself, and with the apoptosis-preventing members Bcl-2 and Bcl-xL using a yeast two-hybrid system. Exhaustive Bax truncations were constructed and their interactions with full-length family members studied. Bax interacted similarly with itself as with the apoptosis-suppressing family members Bcl-2 and Bcl-xL in quantitative two-hybrid studies. A region of 41 amino acids covering the recently discovered BH3 domain of Bax was found to be necessary and sufficient for all interactions of Bax. Bax truncations containing BH3, but lacking BH1 and BH2 homology domains, interacted with the other family members markedly more strongly than full-length Bax, which may reflect conformational changes required for the interactions of full-length Bax. The minimum requirements for Bax homodimerization were found to be the BH3 domain from one Bax molecule and a region covering BH3 plus BH1 from another. We also studied the apoptosis-inducing activity of the Bax truncations upon microinjection of expression plasmids into rat fibroblasts. The BH3 region was required for the apoptosis-inducing activity of Bax, whereas BH1, BH2 and the N-terminus of Bax were dispensable.