: OBJECTIVES: Taking into account the hypothesis that Alzheimer's disease (AD) might be a systemic disease that affects several tissues in the body, the aim of this study was to try to detect the expression of tau-protein in human peripheral blood lymphocytes (PBL) in patients with AD. MATERIAL AND METHODS: Blood samples were obtained from patients with AD (n=16, age 67-98) and from volunteers without psychoneurological pathology (n=10, age 65-78). PBL were isolated on Ficoll-Paque gradient centrifugation. For cell fixation and permeabilization we used a fixative solution (4% formaldehyde and 0.1% glutaraldehyde) and 0.03% Triton X-100. Immunocytochemical detection of tau-protein was carried out by biotin-streptavidin complex method with tau monoclonal antibody (1:100, clone TAU-2, ICN) and universal immunostaining kit IMMU-MARK (ICN). RESULTS: The expression of tau-protein was shown in PBL in absolute majority of AD patients studied. Only in two healthy volunteers a single lymphocyte from many cells (i.e. a smear) demonstrated a very weak-positive immunostaining to tau-protein CONCLUSION: This first demonstration of clear difference in localization of tau-protein in blood lymphocytes between healthy and sick people testifies to the fact that tau-protein could be considered as a promising marker and blood lymphocytes as a suitable sample for life-time diagnosis of AD.