The structure and biological activities of analogs of the bovine neutrophil antibacterial and hemolytic peptide indolicidin, ILPWKWPWWPWRR-amide, where one tryptophan at 4th, 8th, or 11th position has been retained and the others replaced by leucine, have been investigated. All the single tryptophan analogs exhibit antibacterial activity. However, unlike indolicidin, they do not lyse erythrocytes. Structure analysis by circular dichroism spectroscopy indicates that the analogs are unordered in aqueous medium and adopt p-turn structures in trifluoroethanol and micelles. The tryptophan residues in indolicidin appear to be essential for hemolytic activity but not antibacterial activity. The nonspecific biological activities of indolicidin and specific antibacterial activity of single tryptophan analogs suggest that in short peptides, a moth composed of hydrophobic amino acids with the exception of tryptophan, interspaced with proline residues and cationic amino acids at the N or C termini would favor selective antibacterial activity.