Unspecific cellular immune reactivity in 30 patients with squamous cell carcinoma of the head and neck was compared with those in 30 healthy controls. Assays were performed in vitro to evaluate the blastogenic response of lymphocytes to the mitogens PHA (phytohemagglutinin), Con A (Concanavalin A) and PWM (pokeweed mitogen) and to quantify T-rosette-forming lymphocytes. The in vivo assay used was the delayed cutaneous hypersensitivity reaction to DNCB (dinitro-chlor-benzene). Tests were performed in all patients every 4 weeks either for a total of 1 year or until death. Tumor patients were followed up to 5 years. Compared to healthy controls tumor patients demonstrated significant impairment of unspecific immune reactivity. Surgery, chemotherapy, and radiotherapy led temporarily to a further decrease of immune reactivity. There was no correlation between unspecific immune reactivity and tumor stage, course of the disease, and prognosis. It was not possible to give any useful statement for patients with squamous cell carcinoma of the head and neck by determining their unspecific immune reactivity.