Secretory leukocyte proteinase inhibitor (SLPI; also known as antileukoprotease or ALP) is a 11.7 kDa cationic inhibitor of neutrophil elastase and to a lesser extent of cathepsin G. It is locally produced by epithelial cells in the lung, skin and other organs and by PMN and (in mice) by macrophages. In addition to its proteinase inhibitory properties that may serve to protect against proteolytic injury, it was recently shown that SLPI also displays several other functions such as antimicrobial and anti-inflammatory activities. These appear to be independent of its ability to inhibit PMN serine proteinases. SLPI has also been demonstrated to display antibacterial and antifungal activity at concentrations in which SLPI is present in mucosal secretions including those of the lung. Another possible role for SLPI is inhibition of protein disulphide isomerase that is considered essential for invasion of a cell by the Human Immunodeficiency Virus (HIV).Serum/plasma and urine of healthy individuals contains 50-80 ng/ml and 4 ng/ml respectively. Sputum of healthy individuals contains approximately 66 µg/ml.