CD25+ regulatory T cells comprise 5–10% of CD4+ T cells in naïve mice and have been shown in several in vivo murine models to prevent the induction of autoimmune disease and inflammatory disease. Since T cells, which mediate autoimmunity, can through recognition of self-antigens also target tumour cells, it was postulated that CD25+ regulatory cells would also inhibit the generation of immune responses to tumours. Depletion of these cells using CD25-specific monoclonal antibodies has indeed been shown to promote rejection of several transplantable murine tumour cell lines including melanoma, leukaemia and colorectal carcinoma. Results obtained using these models indicate that in the absence of regulatory cells, CD4+ T cells mediate tumour immunity, although the precise mechanisms through which these cells result in tumour rejection have not yet been elucidated. The target antigens recognized by these CD4+ T cells have also not yet been identified. Immunization of mice with tumour cells in the absence of CD25+ regulatory cells does, however, induce immunity against a variety of different tumour cell lines indicating that the target antigen(s) are shared amongst tumours of distinct histological origins. Since CD25+ regulatory cells have been identified in humans, the possibility that the cells inhibit immune responses to shared rejection antigens expressed by human tumours is worthy of investigation.