Some tumor cells have deficits in class I MHC antigen processing, suggesting that T cells exert selective pressure on tumor cells. Previous studies have not revealed increased tumor incidence in mice with deficits in T‐cell immunity, including mice lacking TAP1 (a subunit of the transporter for antigen presentation) or LMP2 (a regulated subunit of the 20S proteasome). The incidence of spontaneous tumors in these mice, however, is too low to assess differences in host resistance to tumors. To increase tumor incidence and better assess the role of systemic expression of TAP1 and LMP2 in responses to tumors, TAP1–/– and LMP2–/– mice were bred with p53–/– mice to create TAP1–/–p53–/– and LMP2–/–p53–/– double knockout mice. Lymphomas and sarcomas (malignant fibrous histiocytoma and angiosarcoma) occurred with high incidence in all p53‐deficient populations. Tumor incidence and death rate were similar in TAP1–/–p53–/– mice and closely matched control TAP1+/+p53–/– mice. Tumor incidence and death rate were slightly accelerated in LMP2–/–p53–/– mice relative to control LMP2+/+p53–/– mice, but the biological significance of this difference was unclear. The relative incidence of lymphomas vs. sarcomas was not significantly altered by variation in TAP1 or LMP2. In conclusion, systemic absence of TAP1 did not alter tumor incidence, while absence of LMP2 was associated with only a slight acceleration of tumor incidence of uncertain significance. These observations are consistent with other evidence that normal T‐cell responses do not effectively limit tumorigenesis. Even though T cells can attack some tumor cells, the ability of tumors to alter their immunogenicity and evade T‐cell surveillance may render the native immune system ineffective at providing a rate‐limiting barrier to tumorigenesis and preventing cancer. © 2001 Wiley‐Liss, Inc.