Neprilysin (CD10) is a transmembrane glycoprotein expressed in a wide variety of tissues and is particularly abundant in kidney. It is also detected in mature neutrophil granulocytes and in several epithelial cell types including liver, prostate, intestine, and breast myoepithelium, as well as some stromal ells and their tumours. It is a common acute lymphocytic leukaemia antigen, which is an important cell surface marker in the diagnosis of human acute lymphocytic leukaemia (ALL). This protein is present on leukaemic cells of pre-B phenotype, which represent 85% of cases of ALL, but is also found in a proportion of T-cell ALL’s (Chu and Arber)¹, (Kaufamann et al)². Since it is expressed by early B, pro-B and pre-B lymphocytes and by lymph node germinal centres, CD10 is of use in haematological diagnosis: It is expressed in the following diseases; angioimmunoblastic T cell lymphoma, Burkitt lymphoma, CML in blast crisis (90%), diffuse large B-cell lymphoma (variable), follicular centre cells (70%), hairy cell leukaemia (10%), and some myelomas (Jaffe et al)^³. It tends to be negative in AML, CLL, mantle cell lymphoma, and marginal zone lymphoma.