The bacteriocin AS-48 is a cationic peptide (7149 Da) having a broad antimicrobial spectrum, encoded by the 68 kb conjugative plasmid pMB2 from Enterococcus faecalis S-48. It is a unique peptide since it has a cyclic structure, which is achieved by the formation of a tail-head peptide bond after ribosomal synthesis (Gálvez et al., 1989; Martínez-Bueno et al., 1994; Samyn et al., 1994). Preliminary CD and calorimetric studies (data not shown) pointed towards a highly helical and very stable three dimensional structure. All the information gathered until now indicates that the target of AS-48 is the cytoplasmic membrane in which it opens channels or pores, leading to dissipation of the proton motive force and cell death, which in some cases is also followed by bacterial lysis (Gálvez et al., 1991). This peptide is a suitable tool for studying protein-membrane interactions, and it also offers promising perspectives for biotechnological applications. Knowledge of the 3D structure of AS-48 is a first step in the conduct of further structure-function studies. Here we report the complete 1H NMR assignment of its proton resonances together with the resulting secondary structure pattern as prerequisites for the determination of a high-resolution 3D solution structure.