Human neutrophil defensins (alpha-defensins, HNP1-3) belong to the family of cationic trisulfide-containing microbicidal peptides. Besides microbicidal, the peptides exert chemotactic, immunomodulating and cytotoxic activity and participate in host defense and inflammation. Azurophilic granules of neutrophils contain Human Neutrophil Peptide (HNP)-1-4 which are highly homologous. The three principal human defensins, HNP 1-3, are unique to neutrophils and account for about 99 percent of the total defensin content of these cells. Measured amount of defensins is 3-5 mg per million human neutrophils. Activation of neutrophils leads to rapid release of defensins. Thus, only one cell type, neutrophils, may be the source of HNP1-3 measured in plasma and other body fluids during infection and inflammation. In normal plasma low levels of HNP are present ranging from undetectable level to 50-100 ng/ml, while in septic conditions the levels of HNP might be elevated to 10 mg/ml and even more. Activation of neutrophils in blood as occurs during clotting, as well as long storage of anticoagulated blood leads to a release of HNP, thus careful plasma sampling is important for possible detection of HNP. Defensins are relatively resistant to proteolysis, low pH and boiling, but have a tendency to bind to a variety of materials, including plastic and proteins. The HNP 1-3 ELISA shows cross reactivity with Rhesus monkey HNP 1-3.