We hypothesized that metabolic products of the Alzheimer β amyloid precursor protein (APP) might be targets for cells of the immune system. To test this hypothesis, peripheral blood lymphocytes from young and old healthy blood donors and patients with Alzheimer's disease were analysed for their responsiveness upon stimulation with amyloid β protein as well as with four other synthetic peptides corresponding to parts of the APP sequence. Stimulation of resting blood lymphocytes from young and old healthy blood donors resulted in IL-2 receptor expression and proliferation in both age groups. In contrast, lymphocytes from the majority of patients with Alzheimer's disease did not proliferate, when stimulated with APP peptides, while their proliferative response to anti-CD3 was unimpaired. This lack of proliferative responsiveness to APP peptides was not due to apoptosis, but could reflect T cell anergy, as it was accompanied by unimpaired IL-2 receptor expression. The results suggest that autoreactive lymphocytes with specificity for metabolic products of APP occur in healthy individuals. These cells may be of relevance for the elimination of potentially amyloidogenic substances. This mechanism could be impaired in patients with Alzheimer's disease.