The peptide, gramicidin A (GrA), has been demonstrated to interact with divalent salts (CaCl2, MgCl2, and ZnCl2) using electrospray ionization mass spectrometry (ESI-MS). The ESI-MS analysis revealed different complexes formed due to the interaction of Val-GrA and Ile-GrA with divalent salts: [Val or Ile-GrA-H+M]+, [Val or Ile-GrA+MCl]+ and [Val or Ile-GrA+M]2+, where M is Ca or Mg or Zn. All these complexes have been subjected to collisionally activated dissociation (CAD). CAD of singly and doubly charged GrA and metal complexes exhibited the losses of water molecules, indicating the ligand preference of GrA. MS/MS and MS3 of [Val or Ile-GrA+MCl]+ resulted in the elimination of chloride ion and water, respectively. The tandem mass spectrometry data of the complex [Val-GrA+MCl]+ suggest that chloride interaction is stronger in the presence of Ca than of Mg and Zn. This study reveals that GrA could interact with Ca, Mg, and Zn in metal ion form as well as in ion pair (MCl) form. The interactions of GrA with Ca support the proposal of a physical basis for the messenger role of Ca (Urry et al., J. Biol. Chem. 1982, 257: 6659–6661). Copyright © 2005 John Wiley & Sons, Ltd.