The monoclonal antibody 31 reacts specifically with secretory leukocyte proteinase inhibitor (SLPI; alsoknown as antileukoprotease (ALP)). SLPI is a 11.7 kDa cationic inhibitor of neutrophil elastase and to alesser extent of cathepsin G. It is locally produced by epithelial cells in the lung, skin and other organs andby Polymorphonuclear leukocytes (PMN) and (in mice) by macrophages.In addition to its proteinase inhibitory properties that may serve to protect against proteolytic injury, it wasrecently shown that SLPI also displays several other functions such as antimicrobial andanti inflammatory activities. These appear to be independent of its ability to inhibit PMN serineproteinases. SLPI has also been demonstrated to display antibacterial and antifungal activity atconcentrations in which SLPI is present in mucosal secretions including those of the lung. Anotherpossible role for SLPI is inhibition of protein disulphide isomerase that is considered essential for invasionof a cell by the Human Immunodeficiency Virus (HIV).The reactivity of the antibody 31 with isolated domains of SLPI was evaluated using domains obtained bycleavage using partial acidic hydrolysis. Due to this fact the monoclonal antibody 31 recognizes otherSLPI cleavage products.