The proton resonances of cytotoxin II from Naja naja oxiana were sequentially assigned in 2D 1H NMR spectra for all of its 60 amino acid residues of both major and minor components of the spectra. The presence of the minor component was shown to be due to a conformational heterogeneity of the cytotoxin. The proton-deuterium exchange rates of amide groups were measured in 2H2O at a pH of 5.0 and at 10°C. Experimental data obtained (d-connectivities, H-NC α-H coupling constants and long-range NOEs) allowed for the determination of the secondary structure of the two cytotoxin conformers. Both conformer structures contain two antiparallel β-sheets. The first β-sheet involves two antiparallel β-strands comprising the residues 2-5 and 10-13. The second sheet involves three antiparallel strands consisting of the residues 20-26, 35-39, and 49-55; its peripheral β-strands are connected by cross-over. The most striking structural difference between these conformers is the nature of the β-turn 6-9, which has trans- and cis-forms of the Val7-Pro8 peptide bond in the major and minor conformer, respectively. The structures of other β-turns, some of which are ascribed to standard types, and the C-termini are almost the same for the both conformers. © 1996 MAEe Cyrillic signK Hayκa/Interperiodica Publishing.