The binding of aluminium ion to [Leu 5]-enkephalin has been investigated by 1H, 13C and 27Al NMR spectroscopy in dimethyl sulphoxide solution at different peptide/metal ratio. Analysis of the spectra suggests that Al 3+ binds at two metal-binding sites. The binding of Al 3+ at the first site involves the Tyr 1CO, and Leu 5COO − groups to give a 2:1 species in a tetracoordinated structure; whereas the binding of Al 3+ at the second site utilizes the NH 2 terminal groups of the tyrosine moiety in a 2:2 species. 27Al chemical shift values strongly suggests that the second type of aluminium atom displays an octahedral environment. On this basis, we discuss our data in terms of the coordination of aluminium with [Leu 5]-enkephalin.