The tumor suppressor p53 is the most frequently mutated gene in human tumors. In response to DNA damage, aberrant growth signals, or chemotherapeutic drugs, p53 is stabilized and induces apoptosis and/or cell cycle arrest. While the mechanisms of p53-dependent apoptosis are not well understood, p53-dependent cycle arrest is primary mediated by the CDK inhibitor p21. p53 is a transcriptional activator and it is not surprising that a majority of p53 mutations occur in the core DNA binding domain and affect DNA binding and transactivation of p53 targets in tumors. We used the capability of p53 to activate transcription for developing a new assay that permits rapid determination of the status of p53 in cancer cell lines of different origin. Our strategy involved using a retrovirus containing a p53-regulated lacZ reporter gene that was introduced into colon and breast tumor cell lines to determine p53 status. Simple staining for beta-galactosidase allowed us to confirm that the colon cancer cell lines LIM1215 and HCT116, as well as the breast cancer cell line MCF7. have wild-type p53, and the colon cancer cell line Caco-2 as well as breast cancer cell lines MDA-MB-435 and MDA-MB-231 have mutant p53. This method may be applied to novel cell lines of any origin with unknown status of p53.