A novel protein family, showing a conserved proregion and a variable C-terminal antimicrobial domain, and named cathelicidin, has been identified in mammalian myeloid cells. The conserved proregion shows sequence similarity to members of the cystatin superfamily of cysteine proteinase inhibitors. Cathelicidins are stored in the cytoplasmic granules of neutrophil leukocytes and release the antimicrobial peptides upon leukocyte activation. Some of these peptides can assume an alpha-helical conformation, others contain one or two disulfide bonds, still others are Pro- and Arg-rich, or Trp-rich. In addition to bacterial killing, some of these peptides exert additional functions related to host defense such as LPS-neutralization and promotion of wound healing.