The p53 protein rapidly accumulates in cells in response to DNA damage, which can trigger apoptosis. This pathway is hypothesized to be important for tumor suppression by p53, as well as for the response of tumors to chemo- or radiotherapy. Implicit in these ideas is that the p53 induction-apoptosis pathway is active in tumor cells in vivo. Because tumor suppression by p53 in mice is markedly tissue-type-dependent, we tested the activity of the pathway in tumors in vivo by inducing tumors in six different tissues and treating tumor-bearing mice with DNA damaging cancer therapeutic agents. In response to treatment, cells from T-cell lymphomas, intestinal adenomas, and mammary tumors rapidly induced p53 and underwent apoptosis. In squamous cell papillomas, p53 was constitutively expressed and was further induced by the treatments, but apoptotic cells were only rarely observed. In treated mice bearing lung or liver adenomas, minimal or no p53 accumulation or apoptosis was observed in the tumor cells. Thus, there is marked variation in the intrinsic ability of autochthonous tumor cells to accumulate p53 and undergo apoptosis. This variation provides one explanation for the tissue specificity of tumor suppression by p53. It also indicates that the role of apoptosis in the response of tumors to therapy varies significantly among tumor types.