On the basis of extraction experiments and cactivity measurements, the extraction constant corresponding to the equilibrium Pb 2? (aq) ? 1ÁSr 2? (nb) ¢ 1ÁPb 2? (nb) ? Sr 2? (aq) occurring in the two-phase waternitrobenzene system (1 = cyclosporin A; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log K ex (Pb 2? , 1ÁSr 2? ) = 0.1 ± 0.1. Further, the stability constant of the 1ÁPb 2? complex in nitrobenzene saturated with water was calculated for a temperature of 25°C: log b nb (1ÁPb 2? ) = 9.2 ± 0.2. Finally, applying quantum chemical DFT calculations, the most probable structure of the proven 1ÁPb 2? cationic complex species was derived. In the resulting complex, the ''central'' cation Pb 2? is bound by four bonding interactions to the corresponding four oxygen atoms of the parent cyclosporin A ligand. The interaction energy, E(int), of the considered 1ÁPb 2? complex was found to be -1016.3 kJ/mol, confirming also the formation of this complex.