Modification of cytotoxin II from Naja oxiana by N-hydroxysuccinimide ester of 2,2,5,5-tetramethylpyrroline-N-oxyl-3-carboxylic acid led to a mixture of spin-labeled derivatives of this toxin. One of these derivatives, purified to a homogenous state by successive ion-exchange chromatography and reverse-phase HPLC, was shown to contain a single label at Lys35 and to be usable in the EPR studying of the cytotoxin II interaction with phospholipid membranes. © 1999 MAEe cyrillic signK "Hayκa/Interperiodica".