Members of the Bcl-2 family of proteins regulate apoptosis, with some of their physiological effects mediated by their modulation of endoplasmic reticulum (ER) Ca²⁺ homeostasis. Antiapoptotic Bcl-xL binds to the inositol trisphosphate receptor (InsP₃R) Ca²⁺ release channel to enhance Ca²⁺- and InsP₃-dependent regulation of channel gating, resulting in reduced ER Ca²⁺, increased oscillations of cytoplasmic Ca²⁺ concentration (Ca²⁺i), and apoptosis resistance. However, it is controversial which InsP₃R isoforms mediate these effects and whether reduced ER Ca²⁺ or enhanced Ca²⁺i signaling is most relevant for apoptosis protection. DT40 cell lines engineered to express each of the three mammalian InsP₃R isoforms individually displayed enhanced apoptosis sensitivity compared with cells lacking InsP₃R. In contrast, coexpression of each isoform with Bcl-xL conferred enhanced apoptosis resistance. In single-channel recordings of channel gating in native ER membranes, Bcl-xL increased the apparent sensitivity of all three InsP₃R isoforms to subsaturating levels of InsP₃. Expression of Bcl-xL reduced ER Ca²⁺ in type 3 but not type 1 or 2 InsP₃R-expressing cells. In contrast, Bcl-xL enhanced spontaneous Ca²⁺i signaling in all three InsP₃R isoform-expressing cell lines. These results demonstrate a redundancy among InsP₃R isoforms in their ability to sensitize cells to apoptotic insults and to interact with Bcl-xL to modulate their activities that result in enhanced apoptosis resistance. Furthermore, these data suggest that modulation of ER Ca²⁺ is not a specific requirement for ER-dependent antiapoptotic effects of Bcl-xL. Rather, apoptosis protection is conferred by enhanced spontaneous Ca²⁺i signaling by Bcl-xL interaction with all isoforms of the InsP₃R.