Tumour rejection may be the cause or consequence of autoreactivity in mice lacking CD25+ regulatory cells. (a) T cells specific for self-antigen (S-ag), induced as a direct result of regulatory T cell depletion, mediate tumour rejection. (b) Immune responses specific for unique tumour antigens (T-ag) are induced as a result of regulatory T cell depletion and mediate tumour rejection. As a consequence of tumour destruction self-antigens are released into a pro-inflammatory environment that facilitates the activation of self-antigen-specific T cells.