Biochemical assay and immunohistochemical staining of neutral endopeptidase were performed on bladder cancer cells. In superficial bladder cancer the enzyme activity and immunohistochemical intensity of staining were high, while invasive bladder cancer showed only a low level of activity. This finding suggests that neutral endopeptidase is expressed at a certain stage of cell differentiation, during the neoplastic process in the bladder. Gene expression is assumed to be closely correlated with this mechanism. From the results of this study neutral endopeptidase will serve as a new tumor marker for bladder cancer as well as acute lymphatic leukemia.