Lymphocytes from Guamanians with amyotrophic lateral sclerosis or parkinsonismdementia and from individuals in good health were studied to determine the possible role of cellular immunity in the pathogenesis of these diseases, which occur at extraordinarily high rates on Guam. Lymphocytes were cultered in the presence of brain material from all three groups. No stimulation was observed in lymphocytes from patients with amyotrophic lateral sclerosis. A low level of stimulation was encountered in lymphocytes from parkinsonism-dementia patients to all brain antigens. While this response could indicate that an immune mechanism plays a role in the pathogenesis of parkinsonismdementia, more likely the response reflects a nonspecific reaction to the massive destruction of central nervous system cells in this disease.