Mammalian antibiotic peptides

Peptide-based compounds containing four invariant cysteine residues which have been oxidized to obtain two intramolecular disulfide bonds are useful as preservatives and in preventing, treating, or ameliorating viral or microbial infection in animals and plants. These compounds, in one embodiment, are of the formula: A.sub.1 -A.sub.2 -A.sub.3 -A.sub.4 -A.sub.5 -C-A.sub.7 -C-A.sub.9 -A.sub.10 -A11 -A12 -C-A14 -C-A16 -(A17 -A18)(1) (SEQ ID NO:4) and the N-terminal acylated, C-terminal amidated or esterified and the cystine-bridged forms thereof wherein A1, A9, A10 and A11 are basic amino acids; A2 and A3 are small amino acids; A5, A7, A12, A14 and A16 are hydrophobic amino acids; and A4 is a basic or a small amino acid; A17 is not present or, if present, is a small amino acid; A18 is not present or, if present, is a basic amino acid.