The contribution of natural cell-mediated cytotoxicity and its modulation with biological response modifiers in antitumour immunity was evaluated in patients with precancers and cancers of the oral cavity. The results were compared to those in normal controls. Both groups of patients showed highly significant depression of the natural killer cell activity. This depression was stage-related, suggesting an influence of tumour or tumour products on this activity. Depression in augmentation of this cytotoxicity was evident in both patient groups with cancers and precancers, suggesting a defect in the response of killer cells to biological response modifiers for reasons other than the tumour per se.