A monoclonal antibody to enkephalinase (membrane metalloendopeptidase E.C. 3.4.24.11) raised by immunizing mice with rabbit renal cortical cells (mAb 85A2) was used to assess the tissue distribution of the enzyme in adult rats. When incubated with enkephalinase purified from renal brush border or with solubilized brush border proteins in the presence of antimouse IgG, the mAb specifically precipitates active enkephalinase as assessed by thiorphan inhibitable hydrolysis of [3H]-(D.Ala2, Leu5)]enkephalin. Immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis studies of radiolabeled brush border proteins indicate that mAb 85A2 binds specifically an antigen with an apparent molecular weight of 90 kilodaltons, corresponding to published values for enkephalinase. This antibody was used to localize enkephalinase by autoradiographic and immunofluorescence techniques. Immunohistochemical data show intense expression of this ubiquitous enzyme in brain, kidney, thyroid, parts of intestine, lung, seminal vesicle, and prostate. Specific areas of the brain contain more enkephalinase than others. Similarly, specific intense localization demonstrated in pulmonary alveolar cells and along the brush borders of the gut and the pars recta of the renal proximal tubule indicates that only select cell populations and/or cell compartments express this enzyme.