Peripheral blood T cell subsets from 99 patients with nasopharyngeal carcinoma (NPC), 38 age-matched healthy subjects, and 20 patients with head and neck cancer other than NPC were characterized by reactivity with monoclonal antibodies. Patients with NPC showed much lower values of percentage and absolute number of common T cells (OKT3+) and helper cells (OKT4+) (p less than 0.01) than the control groups. The percentage of suppressor cells (OKT8+) was found to be higher in NPC patients than in healthy persons (p less than 0.001), but the absolute number was still lower in NPC patients. A selective decrease in the ratios of OKT4+ to OKT8+ cells was found in NPC patients (p less than 0.001). The ratios were correlated inversely to the clinical stage (r = 0.294, p less than 0.01). After negative selection by complement-mediated cytolysis, the remaining cells were stimulated with phytohemagglutinin. Net counts of isotope incorporation were smaller in all T cell subsets of NPC patients (p less than 0.001). These data indicate that the depressed cell-mediated immunity in NPC patients is due to inadequate number and functions of T cell subsets and a relative increased proportion of suppressor T cells.