BACKGROUND AND OBJECTIVE: Creutzfeldt-Jakob disease typically affects older patients, yet victims of new-variant Creutzfeldt-Jakob disease (nvCJD) are unusually young. Because the cellular prion protein PrP(C) is required for disease development, we investigated age-dependent variability in cell surface PrP(C) expression on various subclasses of human peripheral blood leukocytes (PBL) as a possible susceptibility factor. DESIGN AND METHODS: Three age groups of healthy individuals (mean ages: 6, 33 and 68) were studied by two-color FACS analysis of PBL with fluorescent monoclonal antibodies to PrP(C) and to the lineage markers CD3, CD19, CD4, and CD8. For each subclass marker, surface PrP(C) levels were expressed as mean fluorescence intensity ratios (MFIR) by dividing the geometric mean of the fluorescence of each test antibody by the geometric mean of its isotype-matched control antibody. PrP(C) expression levels in each age and lineage group were compared using appropriate non-parametric tests. RESULTS: We found significant age-related differences in PrP(C) expression on lymphocytes (p=0. 0004). The elderly expressed significantly higher levels than children (p=0.0006) and adults (p=0.0009). PrP(C) expression was also significantly higher in CD3(+) compared to CD19(+ )(p=0.0004) and in CD8(+) compared to CD4(+ )lymphocytes (p=0.0044). INTERPRETATION AND CONCLUSIONS: If PrP(C) expression on PBL were a significant susceptibility factor for nvCJD, young persons would display higher levels. Instead, the elderly expressed the highest amounts of PrP(C) on PBL. This argues against the hypothesis that variability in cell surface expression of PrP(C) in PBL contributes to the exquisite susceptibility of the young to nvCJD.