Marker release from liposomes induced by the cytolytic protein Cerebratulus lacteus toxin A-III was studied. No phospholipid specificity was apparent, but the sensitivity of liposomes to A-III varied with the membrane fluidity. With dioleylphosphatidylcholine liposomes, complete release occurred at 10-20 micrograms toxin per ml, depending on marker size. Kinetic experiments showed that release was rapid and exhibited no lag phase. The diameter of the A-III produced membrane lesion must exceed 90 A, as tetrameric Concanavalin A is quantitatively released from A-III treated liposomes.