Patients with the syndrome of MCI (n=22) and a control group (31 people without signs of mental and neurological diseases) were tested for the degree of hemolysis of erythrocytes (RBC), (membrane structure markers, acetylcholinesterase (AChE) activity in RBC), the level of lipid peroxidation (LPO) (content of malondialdehyde - MDA), microviscosity of surface areas of the membrane using two spin probes localized in the bulk lipids (s1) and in adjacent to the proteins regions of lipid bilayer (s2). There was no difference between the level of spontaneous hemolysis in the group with MCI and the control group which indicated the absence of changes in the mechanical resistance of the RBC membrane. Significant between- group differences were identified for s2 (the increase by >40% in MCI). A trend toward the increase in MDA level in MCI was found as well. There was a decrease by approx. 30% in the Vmax and Km for AChE reaction that indicated the changes in properties or amount of this enzyme. The value of Vmax/Km that determined the real activity of AChE was similar in both groups. No correlations between parameters of membrane structural-functional state that are characteristic of normalcy were noted in the patients with MCI. This finding demonstrated the changes in structural-functional regulation in RBC membrane in elderly people with MCI. Membrane structure status of erythrocytes in MCI holds a special position and differs both from normal ageing and developed Alzheimer's disease. The s1/s2 ratio is recommended as a membrane "marker"for identification of MCI.