Oncogenesis is a process resulting from genetic events which cause loss of growth control or inhibition of appropriate cell death. The Bcl-XL protein is a recently discovered member of the bcl-2 family which has been shown to protect cells from some forms of programmed cell death, but has not yet been implicated in the genesis of human carcinomas. In this report we explore the role of Bcl-XL overexpression in protecting cancer cells from p53-mediated apoptosis. Increased levels of Bcl-XL were found in a subset of primary human breast carcinomas, as well as in the breast cancer line, T47D. T47D cells were then transfected with a temperature-sensitive mutant of the tumor suppressor p53 (p53ts). Although many tumor cell lines undergo apoptosis when p53 is expressed, the T47D transfectants remained viable at temperatures permitting wild-type p53 phenotype. This suggested that endogenous Bcl-XL could protect cancer cells from p53-mediated apoptosis. To test this hypothesis, murine erythroleukemia cells were transfected with bcl-XL and p53ts. While cell lines expressing p53 alone rapidly died, those cells co-expressing Bcl-XL survived. These results demonstrate that Bcl-XL is capable of protecting cells from p53-mediated apoptosis, and suggest a possible mechanism by which tumors expressing Bcl-XL are able to partly overcome the tumor suppressor functions of p53.