Cathelicidins are a family of antimicrobial proteins predominantly found in the peroxidase-negative granules of neutrophils. During bacterial infections, the life span of neutrophils is regulated by various pathogen- and host-derived substances. The cathelicidins are synthesized as preproproteins. Within the neutrophils, they are stored in granules as inactive proforms after removal of the signal peptide. The active biologic domains of the cathelicidins generally reside in the C-terminus. The C-terminal antimicrobial peptides are activated when cleaved from the proforms of the cathelicidins by serine proteases from azurophil granules. Human cationic antimicrobial protein (hCAP)18 is the only human cathelicidin identified to date. hCAP18 (18 kD) is a major protein in specific granules of neutrophils, but it is also present in subpopulations of lymphocytes and monocytes, in squamous epithelia (of the mouth, tongue, esophagus, cervix, and vagina), pulmonary epithelium, keratinocytes in inflammatory skin diseases and in the epididymus. The antibacterial C-terminus of hCAP-18, LL37 (37 aminoacids), has been shown to exert broad antimicrobial activity towards gram-negative as well as gram-positive bacteria and to have synergistic antibacterial effects with the defensins. For instance deficiency in saliva LL37 accords with occurrence of periodontal disease in patients with morbus Kostmann. LL-37 does not only kill bacteria, but can also modulate (suppress) neutrophil apoptosis via the activation of FPRL1 and P2X7 in bacterial infections. Suppression of neutrophil apoptosis results in the prolongation of their life span, and may be advantageous for host defense against bacterial invasion. Moreover it functions as a chemotactic agent for neutrophils, monocytes and T cells. LL-37 is markedly resistant to proteolytic degradation and to a limited extent also cytotoxic towards mammalian cells.