Abstract Human membrane-bound neutral arylamidases were solubilized from small intestinal mucosa, lung, kidney, liver and placenta with trypsin. These five membrane-bound neutral arylamidases were identified by polyacrylamide geldisc electrophoresis. The heat sensitivity of each enzyme was in the order, liver and placenta > kidney > lung > small intestine. This order correlates with that of electrophoretic mobility, except for the placental membrane-bound neutral arylamidase. Five membrane-bound neutral arylamidases have the same molecular weight, 240 000, as estimated by Sephadex G-200 gel filtration. The five membrane-bound neutral arylamidase have very similar K M values (8.7 × 10 -5 M towards l-alanyl-β-naphthylamide), optimal pH values, hydrolysis ratios towards l-alanyl-β-naphthylamide and l-leucyl-β-naphthylamide, and sensitivities of inhibition by chelators or amino acids. These results suggest that the multiple forms of membrane-bound neutral arylamidase found in five different human organs are organ-specific isoenzymes.