Neutral endopeptidase from human or bovine tissues retains enzymatic activity following electrophoresis and immobilization in polyacrylamide gels. Infiltration of the gel with a fluorogenic substrate permits identification of the active enzyme by fluorescence associated with a distinct protein band. This technique both separates and identifies the enzymatically active species from a crude cell membrane fraction or from partially purified extracts that contain contaminating proteins. Enzymatic activity is quantitated by photographing the fluorescent bands and scanning the negatives with a laser densitometer. Because as little as 25 ng of enzyme can be detected by this method, it could be used where the amount of material is limited.