Cathelicidins are a family of antimicrobial proteins predominantly found in the peroxidase-negative granules of neutrophils. 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. The antibacterial C-terminus of hCAP-18, LL37 (37 amino acids), 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. 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. LL-37 was demonstrated to be present in plasma in levels from 25 - 250 ng/ml and to be enhanced in infectious diseases.