We estimated membrane fluidity by the (ESR) spectroscopy and the expression of membrane P-selectin and glycoproteins GP Ib&agr; and GP IIb/IIIa by flow cytometry in platelets (plts) from 12 AD sufferers. In AD patients membrane fluidity was significantly increased at two different depths (p< 0.05 or less). Platelet reactivity was significantly decreased, as reflected by reduced expression of GP Ib&agr; in the resting (p< 0.0001) and activated (p< 0.005) platelets, as well as the expression of P-selectin and &bgr;3subunit of GP IIb/IIIa in activated platelets (p< 0.0001;p< 0.04).