Cellular immunity was assessed in patients with operable squamous cell cancer of the head and neck using in vivo skin tests and in vitro lymphocyte stimulation tests. An expansion of a previous study continued to show that 30 per cent of patients with T1N0M0 lesions were DNCB-negative and that with more advanced lesions there was further impairment. A similar finding was observed in the blastogenic response to phytohemagglutinin and concanavalin A but not pokeweed mitogen. Overall, 40 per cent of patients with resectable cancer had a significant depression of the blastogenic responses to conconavalin A and phytohemagglutinin. This depression ranged from 15 per cent in patients with T1N0M0 lesions to 71 per cent in those with T3N0M0 lesions. Although this depression was more severe in patients with palpable cervical node metastases, it was related more to the size of the primary tumor than to the nodes per se. An exception occurred in patients with large fixed nodes in whom the depression of lymphocyte stimulation was most severe. The absolute T-cell count was also depressed in patients with head and neck cancer. This depression parallelled the lymphocyte stimulation results with phytohemagglutinin and conconavalin A and was progressive with increasing stage of disease. A correlation exists between DNCB negativity and early recurrence and shortened survival. Clinical follow-up study is too short to assess the correlation of in vitro immune function with these clinical prognostic factors.