Norepinephrine-stimulated skin secretions were obtained from male specimens of the South American bullfrog, Leptodactylus pentadactylus and shown to contain two peptides that inhibited the growth of microorganisms. The primary structure of a previously undescribed peptide, termed pentadactylin, was established as Gly-Leu-Leu-Asp-Thr-Leu-Lys-Gly-Ala-Ala-Lys-Asn-Val-Val-Gly-Ser-Leu-Ala-Ser-Lys-Val-Met-Glu-Lys-Leu.NH2. The second peptide, which differs from pentadactylin by eight amino acid residues, is identical to fallaxin previously isolated from skin secretions of the Caribbean mountain chicken frog L. fallax. Pentadactylin inhibited the growth of reference strains of both Gram-negative bacteria (Escherichia coli, Enterobacter cloacae, Klebsiella pneumoniae, Pseudomonas aeruginosa) and Gram-positive bacteria (Staphylococcus aureus, Staphylococcus epidermidis, Enterococcus faecalis, Streptococcus group B) but potencies were relatively low (MIC values in the range 25-200 microM). The peptide showed very low hemolytic activity against human erythrocytes (LD50>400 microM).