THE Bcl-2 family of proteins regulate programmed cell death by an unknown mechanism(1). Here we describe the crystal and solution structures of a Bcl-2 family member, Bcl-x(L) (ref. 2). The structures consist of two central, primarily hydrophobic alpha-helices, which are surrounded by amphipathic helices. A 60-residue loop connecting helices alpha 1 and alpha 2 was found to be flexible and non-essential for anti-apoptotic activity. The three functionally important Bcl-2 homology regions (BH1, BH2 and BH3)(3-5) are in close spatial proximity and form an elongated hydrophobic cleft that may represent the binding site for other Bcl-2 family members. The arrangement of the alpha-helices in Bcl-x(L) is reminiscent of the membrane translocation domain of bacterial toxins, in particular diphtheria toxin and the colicins(6). The structural similarity may provide a clue to the mechanism of action of the Bcl-2 family of proteins.