A DNA microarray analysis identified the BH3-only BCL-2 family member, BIK/NBK, as a transcript that is upregulated during induction of apoptosis by oncogenic E1A. E1A depended on wild-type p53 to induce BIK and activate the death program. Further, p53 independently induced BIK RNA and protein, and BIK alone stimulated cell death in p53-null cells, dependent on the activation of caspases. BIK function, however, was abrogated by a disabling point mutation within the BH3 domain. Collectively, these results argue that BIK is a downstream apoptotic effector of p53 in response to a physiological p53-mediated death stimulus provided by E1A. Elevated BCL-2 functioned downstream of p53 and BIK induction to inhibit the E1A death pathway, with the ratio of anti-apoptotic BCL-2 and pro-apoptotic BIK determining cell death or survival in E1A-expressing cells. Cells expressing BCL-2 or treated with the pan caspase inhibitor, zVAD-fmk, allowed accumulation of high levels of cytotoxic BIK compared to control cells. Of note, a significant fraction of either ectopic or endogenous BIK was found associated with the endoplasmic reticulum, suggesting that this organelle, in addition to mitochondria, may be a target of BIK function.