Neutrophil elastase (NE) is a causative factor of various inflammatory diseases, including emphysema and rheumatoid arthritis. Induced by endotoxins NE may play a role in the regulation of the antiproteinase (including SLPI and Elafin/SKALP) screen in the lung and the formation of a protective surface at the epithelial site. The antibody labels neutrophils and neutrophil precursors, and also reacts (although more weakly) with a minor population of blood monocytes. Neoplastic cells in about 70% of acute myeloid leukaemias are labelled by the antibody, and it is a useful supplement to other antibodies to myeloid markers in the differential identification of acute myeloid leukaemias and extramedullary myeloid cell tumours. Differential identification is aided by the results from a panel of antibodies and traditional histological and enzyme cytochemical methods. Interpretation must be made within the context of the patient’s clinical history and other diagnostic tests by a qualified pathologist.