T he hypothesis that an immune response is directed to components of brain tissue in Huntington's disease (HD) was examined. 27 patients with HD, 21 children of affected parents who showed no signs of disease, 6 spouses of HD patients, and 40 patients with other neurologic diseases were investigated. Leukocyte migration inhibition to antigens prepared from brain tissue was demonstrated in 26 HD patients and in 9 children. None of the six spouses reacted. 13 patients with other neurologic conditions (including 60% of patients after cerebrovascular accidents) reacted to rat brain extract. Fractionation of brain tissue indicated that the material responsible for reactions in these patients differed from that in HD. The material that elicited hypersensitivity in HD was not species specific, since it was found in rat, calf, human, and rabbit brain. It was not present in rat kidney, liver, or heart. Preliminary characterization showed that the material was protein in nature, contained sugar and lipid, and was heat labile. The finding of hypersensitivity in about half the unaffected children of HD parents suggests that it may precede the clinical onset of disease and serve as a marker for the gene.