Tumour-induced expansion of regulatory T (T_Reg) cells is an obstacle to successful cancer immunotherapy. The potential benefit of T_Reg-cell depletion through the interleukin-2 receptor is lost by the concurrent elimination of activated effector lymphocytes and possibly by the de novo induction of T_Reg-cell replenishment. In theory, the functional inactivation of T_Reg cells will maintain them at high numbers in tumours and avoid their replenishment from the peripheral lymphocyte pool, which has the capacity to further suppress the effector lymphocyte anti-tumour response.