Antioxidative properties of a series of morpholinium quaternary ammonium salts (PPMA-n; alkyl chain carbon number n = 7, 9, 11, 13 and 15) were determined. PPMA-n compounds were incorporated into erythrocyte membranes in sublytic concentrations, erythrocytes were then UV-irradiated and efficiency of antioxidative protection of PPMA-n determined spectrophotometrically by measuring the concentration of malone dialdehyde considered as an indicator of the lipid peroxidation. The antioxidative activity of PPMA-n was found to increase with elongation of their alkyl chain. Protection of erythrocyte membrane lipids against oxidation by PPMA-15 was found to be about four times better than that by PPMA-7. The results obtained indicate that the compounds studied can be used at micromolar concentrations as effective antioxidants and that this antioxidative activity is directly related to compound`s lipophilicity. Since lipophilicity is, among others, a factor determining the depth of incorporation into membrane, it is rather obvious that the deeper a molecule incorporates the better its antioxidative protections. In order to confirm this conclusion, the measurements of changes in membrane fluidity caused by incorporating PPMA-n were performed using fluorescent probes. Both the probes (TMA-DPH and DPH) showed that there is a relation between alkyl chain length and change in anisotropy.