<jats:p> Caerin 4.1 (GLWQK5IKSAA10GDLAS15GIEVG20IKS-NH2) is an antibiotic peptide isolated from the Australian tree frog Litoria caerulea. Unlike caerin 1.1, the major peptide isolated from this species, caerin 4.1 has a narrow spectrum of antibiotic activity, e.g. it shows selective activity against Pasteurella haemolytica and Escherichia coli. Caerin 4.1 consists of 23 amino acid residues and is comparable in size with other wide-spectrum antibiotic peptides isolated from Australian amphibians, e.g. caerin 1.1 and maculatin 1.1. An n.m.r. study in trifluoroethanol/water indicates that caerin 4.1 forms an amphipathic α-helix with distinct hydrophilic and hydrophobic zones. Two regions of well defined helicity (from Gln4 to Ala10 and from Ile17 to Ile21) are separated by a central helical region of greater conformational variability. The enhanced disorder in this region arises from the presence of two central glycine residues at positions 11 and 16. However, the degree of disorder and hence flexibility is much less than in caerin 1.1 where central proline residues are present instead. This reduced central flexibility may account for the narrow spectrum of biological activity of caerin 4.1, i.e. because biological membranes of the various bacteria have different composition and topology, their optimal interaction with the relatively rigid caerin 4.1 peptide is not possible.