From freeze dried mucus of the Antarctic nemertine Parborlasia corrugatus we have isolated 10.3kDa basic (pI>9.0) cytolytic protein, referred to as parborlysin. Although the purified protein sample was homogeneous by reversed phase HPLC chromatography profiles and several gel electrophoretic techniques, N-terminal sequence and mass spectrometric analyses revealed that it consisted of few very similar isotoxins. The N-terminal sequence of the parborlysin sample shows a high degree of homology with the sequence of cytolysin A-III from the heteronemertine Cerebratulus lacteus. Parborlysin in micromolar concentration range disrupts mammalian erythrocytes with an apparent detergent mode of action. Hemolytic activity was inhibited by preincubation of parborlysin with pure phosphatidic acid or with rather high concentrations of small unilamellar vesicles composed of phosphatidylcholine (PC)/phosphatidyglycerol, PC/phosphatidylinositol, and PC/phosphatidylserine. Osmotic protectants as large as 3000Da failed to protect red cells from lysis induced by parborlysin. Further structural and pharmacological analysis of the heteronemertine cytolysins may provide new insights regarding the mechanisms by which some water soluble proteins are able to penetrate into lipid membranes and form pores or, acting as detergents, disrupt their normal structure and function. © 2003 Elsevier Science Ltd. All rights reserved.