Modifications of the peptide pyrrhocoricin permit the production of a variety of anti-bacterial or anti-fungal peptides having general formula R¹-Asp-Lys-Gly-X-Y-Leu-Pro-Arg-Pro-Thr-Pro-Pro-Arg-Pro-Ile-Tyr-X-Y-R² SEQ ID NO: 1 or multimeric compositions containing more than a single peptide of that formula. These peptides may be straight chain or cyclic peptides, and may contain one or more non-cleavable bonds. These peptides are characterized by anti-bacterial or anti-fungal activity and metabolic stability in mammalian serum. These peptides are useful in anti-bacterial or anti-fungal pharmaceutical compositions and for further drug development or identification of other antibiotic or anti-fungal compounds.