The tumor suppressor gene p53 maintains the integrity of the genome by stimulating apoptosis in cells that have sustained DNA damage. The p53 gene is frequently altered in human cancers, including breast cancer, and such alterations are thought to result in genomic instability and aneuploidy, 2 hallmarks of anaplastic cells. We used radiation as a DNA-damaging agent to test the role of p53 in controlling apoptosis propensity in pre-neoplastic mammary lesions in mice. Four different pre-neoplastic mammary outgrowth lines, D1, TM2H, TM4 and TM12, were maintained by serial transplantation in the cleared mammary fat pads of syngeneic BALB/c mice. These lines have known alterations in p53 expression: TM12 has normal expression, D1 over-produces wild-type protein, TM2H contains a deletion resulting in a null phenotype and TM4 produces a mutant protein. Mice bearing the various outgrowths were irradiated with 5 Gy, and apoptosis was scored by examination of histological sections prepared from the outgrowths 6 hr after irradiation. The TM12 outgrowths, but not the TM2H outgrowths, exhibited radiation-induced apoptosis. The D1 and TM4 lines expressed radiation-induced apoptosis while the fat pads were being repopulated; however, the induced apoptosis declined to low levels as the mice aged. These results are consistent with the hypothesis that normal p53 function is important if mammary cells with DNA damage are to be deleted by apoptosis.