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The majority of reports indicate that blood platelet membrane fluidity is increased in Alzheimer's disease (AD) patients. The increased membrane fluidity implies that platelet membrane receptors are less exposed to the external environment. To verify this hypothesis we tried to estimate platelet membrane fluidity at its two different depths and receptors' exposure after platelet activation in 12 AD sufferers. Platelet membrane fluidity was measured by the EPR with the use of 2 spin-labelled markers. In AD patients both "near surface" (p < 0.04) and "deeper depth" (p < 0.005) fluidity was significantly increased. The exposure of platelet granule membrane protein--P-selectin and membrane glycoprotein receptors: alpha subunit of glycoprotein Ib and beta3 subunit of GP complex IIb/IIIa were measured by flow cytometry with the use of human platelet monoclonal antibodies labelled with fluorescein and ficoerithrin. The exposure of GPIb alpha subunit was significantly decreased both in resting state (p < 0.0001) and after thrombin activation (p < 0.005). In thrombin-activated platelets the expression of P-selectin and beta3 subunit of fibrinogen receptor were also significantly decreased (p < 0.00001 and p < 0.04, respectively). The authors conclude that both platelet membrane fluidity and receptor exposure might serve as an adjunct marker of in vivo AD diagnosis.