A role for T cells in immune surveillance has long been suggested1, but the lack of high tumour incidence in T cell-deficient nude mice represented a challenge to this hypothesis. The discovery of natural killer (NK) cells, which can lyse tumour cells without any previous sensitization, and high NK levels in nude mice, suggested that these cells may constitute a first line of defence against spontaneously arising tumours in vivo2. Reports3–5 of a selective NK deficiency and normal T-cell function in C57BL/6 mice carrying the beige mutation (bg/bg) suggested that these mice might be useful for assessing the relative importance of T-cell and NK-cell systems in immune surveillance. However we report here that the deficiency of beige mice is not restricted to NK cells. The generation of cytotoxic T lymphocytes (CTLs) in response to alloimmune challenge in vivo or in vitro was markedly impaired in beige mutants. Thus our results do not support the suggestion3,5 that beige mice might be useful as a model of a selective NK deficiency.