The objective of the research study was to determine the antioxidant activity of aqueous solutions of extracts from red current and cranberry with regard to the erythrocytes membrane, and the mechanism responsible for that activity. The polyphenolic composition of the extracts was determined using an ultraperformance liquid chromatography method with diode array detection and mass analysis (UPLC-DADMS). Spectrophotometrically was determined a partition coefficient for compounds contained in extracts between the organic and aqueous phases. The antioxidant activity of extracts towards the erythrocytes membrane was analyzed using a fluorescence method on the basis of a quenching degree of DPH-PA probe fluorescence, whereas the oxidation of the erythrocytes membranes was induced by an AAPH compound. The fluorescence spectroscopy was also used to determine the location of phenolic compounds, contained in the extracts, in the erythrocyte membrane. It was found that the red currant and cranberry fruits were rich in polyphenols, in particular in anthocyanins and procyanidins. With the use of fluorescence probes, it was proved that polyphenolic compounds contained in the extracts did not penetrate into the hydrophobic region of the membrane; the absence of any changes in the fluorescence anisotropy confirmed it. However, they bound to the region of polar heads on the membrane surface and changed their packing as evidenced by the decrease in the generalized polarization. It was proved that the polyphenols in the extracts efficiently protected the erythrocytes membrane against harmful activity of free radicals induced by the AAPH compound in the aqueous environment. Furthermore, it was proved that the antioxidant activity of red currant extract (IC₅₀ = 4.59 μg/ml) was comparable to that of a standard antioxidant, i.e. of Trolox^® (IC₅₀ = 3.9 μg/ml) and that it was better than the activity of cranberry extracts (IC₅₀ = 12.02 μg/ml).