5-(n-Alk(en)yl) resorcinols can induce potassium release from liposomes and erythrocytes. The results suggest that 5-(n-pentyl)resorcinol can induce a specific permeability to protons as well as to potassium and other small molecules. The highest permeability changes were found in the presence of 5-(n-pentadecyl)resorcinol and alkenyl resorcinols. Orcin and resorcin were without effect. The size of permeant as investigated by turbidity measurements indicated that Ca2+ and Mg2+ cannot pass through the alkyl resorcinol-modified membrane but can pass through the alkenyl resorcinol-modified membrane. It was observed that alkenyl resorcinol at a concentration of 15 microM induced not only potassium release but also lysis of erythrocytes.