The high antioxidant capacity of chlorogenic acid (CGA) in respect to biological systems is commonly known, though the molecular mechanism underlying that activity is not known. The aim of the study was to determine that mechanism at the molecular and cell level, in particular with regard to the erythrocyte and the lipid phase of its membrane. The effect of CGA on erythrocytes and lipid membranes was studied using microscopic, spectrophotometric and electric methods. The biological activity of the acid was determined on the basis of changes in the physical parameters of the membrane, in particular its osmotic resistance and shapes of erythrocytes, polar head packing order and fluidity of erythrocyte membrane as well as capacity and resistivity of black lipid membrane (BLM). The study showed that CGA becomes localized mainly in the outer part of membrane, does not induce hemolysis or change the osmotic resistance of erythrocytes, and induces formation of echinocytes. The values of generalized polarization and fluorescence anisotropy indicate that CGA alters the hydrophilic region of the membrane, practically without changing the fluidity in the hydrophobic region. The assay of electric parameters showed that CGA causes decreased capacity and resistivity of black lipid membranes. The overall result is that CGA takes position mainly in the hydrophilic region of the membrane, modifying its properties. Such localization allows the acid to reduce free radicals in the immediate vicinity of the cell and hinders their diffusion into the membrane interior.