Cereal grain resorcinolic lipids (5-n-alk(en)ylresorcinols) at micromolar concentrations are able to protect the erythrocyte membrane against hydrogen peroxide-induced lipid oxidation. The antioxidative effect is dependent upon chain length of alkylresorcinol molecules. The C15:0 homolog (IC50 of 10 microM) exhibited strongest activity whereas for long chain homologs (C19:0 and C23:0) IC50 values were higher, 32.5 and 59 microM, respectively. The protective effect of alkylresorcinolic antioxidants was also dependent on their incorporation into the membrane, that is governed by their water-membrane partition coefficient. The results obtained show that alkylresorcinols should be recognized as hydrophobic, membrane-localised antioxidants.