beta-Secretase activity is the rate-limiting step in Abeta peptide production from amyloid precursor protein. Abeta is a major component of Alzheimer's disease (AD) cortical amyloid plaques. beta-Secretase activity is elevated in post mortem brain tissue in AD. The current study investigated whether beta-secretase activity was also elevated in peripheral blood platelets. We developed a novel fluorimetric beta-secretase activity assay to investigate platelets isolated from individuals with AD (n=86), and age-matched controls (n=115). Platelet membrane beta-secretase activity (expressed as initial rate) varied over fourfold between individuals, raising important questions about in vivo regulation of this proteolytic activity. Nonetheless, we identified a significant 17% increase in platelet membrane beta-secretase activity in individuals with AD compared to controls (p=0.0003, unpaired t-test). Platelet membrane beta-secretase activity did not correlate with mini-mental state examination (MMSE) score in the AD group (mean MMSE=17.7, range 1-23), indicating that the increase did not occur as a secondary result of the disease process, and may even have preceded symptom onset.