Bcl-2 contributes to the pathophysiology and therapeutic resistance of chronic lymphocytic leukemia (CLL). As such, developing inhibitors of this protein based on thorough understanding of its mechanism of action is an active and promising area of inquiry. One approach centers on agents (e.g., ABT-737) that compete with proapoptotic members of the Bcl-2 protein family for binding in the hydrophobic groove formed by the BH1-BH3 domains of Bcl-2. Yet another region of Bcl-2, the BH4 domain, also contributes to the antiapoptotic activity of Bcl-2 by binding to the inositol 1,4,5-trisphosphate receptor (IP(3)R) Ca(2+) channel and thus inhibiting IP(3)-dependent Ca(2+) release from the endoplasmic reticulum. Here we report that a novel synthetic peptide, modeled after the Bcl-2-interacting site on the IP(3)R, binds to the BH4 domain of Bcl-2 and functions as a competitive inhibitor of Bcl-2-IP(3)R interaction. By disrupting Bcl-2-IP(3)R interaction this peptide induces an IP(3)R-dependent Ca(2+) elevation in lymphoma and leukemia cell lines and in primary CLL cells. The Ca(2+) elevation evoked by this peptide induces apoptosis in CLL cells, but not in normal peripheral blood lymphocytes, suggesting involvement of the Bcl-2-IP(3)R interaction in the molecular mechanism of CLL and indicating the potential merit of targeting this interaction therapeutically.