Binding of neurotoxin II Naja naja oxiana derivatives containing one spin label at various positions (Leu 1, Glu 2, Lys 15, Lys 25, Lys 26, His 31, Lys 44 and Lys 46) to purified solubilized acetylcholine receptor protein (AchR) from Torpedo marmorata was studied by EPR techniques. AchR interaction with several dansylated neurotoxin II derivatives was followed by difference fluorescence spectroscopy. A series of neurotoxin II p-azidobenzoyl derivatives were prepared and in three of them modified lysine residues were identified. In combination, spectroscopic data and photolabeling implicate a considerable area of the neurotoxin in association with AchR. Rigidity of the neurotoxin II conformation allowed to regard its binding surface as a mould of the AchR corresponding site and to estimate the minimal size of the latter. Conformation of the long-chain neurotoxins and their binding to AchR are briefly discussed basing on the 1H and 19F NMR studies of neurotoxin I Naja naja oxiana, toxin 3 Naja naja siamensis and its acetylated or trifluoroacetylated derivatives, as well as on Achr interaction with the derivatives spin labeled at Lys 27 and His 71.