Cytotoxin II from the venom of the Central-Asian cobra Naja oxiana spin-labeled at Lys35 (SLCT II) was studied by ESR spectroscopy in aqueous solution and upon interaction with phospholipid vesicles from egg phosphatidylcholine or its mixture with dimyristoylphosphatidylglycerol (molar ratio 9 : 1). The distribution of SLCT II between the aqueous and lipid phases depended on the toxin and lipid concentrations and on the solution ionic strength. It was analyzed using the modified Gouy-Chapman equation that takes into account different charges of the cytotoxin in solution and in membrane. The analysis revealed two states of the cytotoxin-lipid complex. The first state corresponds to monomeric SLCT II hydrophobically interacting with the lipid membrane [a binding constant of (8 ±3) x 103M-1] and carrying the charge of 4.4 ± 0.3. On the basis of these parameters and the spatial structure of cytotoxin II in dodecylphosphocholine micelles, we concluded that the cytotoxin is mainly incorporated into the region of polar groups of the lipid bilayer. The second state of SLCT II is realized at high cytotoxin concentrations in the membrane and corresponds to the formation of toxin-lipid complexes that destruct the membrane bilayer structure.