Rhesus monkey bone marrow expresses a cathelicidin whose C-terminal domain comprises a 37-residue alpha-helical peptide (RL-37) that resembles human LL-37. Like its human counterpart, RL-37 rapidly permeabilized the membranes of Escherichia coli ML-35p and lysed liposomes that simulated bacterial membranes. When tested in media whose NaCl concentrations approximated those of extracellular fluids, RL-37 was considerably more active than LL-37 against staphylococci. Whereas human LL-37 contains five acidic residues and has a net charge of +6, rhesus RL-37 has only two acidic residues and a net charge of +8. Speculating that the multiple acidic residues of human LL-37 reduced its efficacy against staphylococci, we made a peptide (LL-37 pentamide) in which each aspartic acid of LL-37 was replaced by an asparagine and each glutamic acid was replaced by a glutamine. LL-37 pentamide's antistaphylococcal activity was substantially greater than that of LL-37. Thus, although the precursor of LL-37 is induced in human skin keratinocytes by injury or inflammation, its insufficiently cationic antimicrobial domain may contribute to the success of staphylococci in colonizing and infecting human skin.