Purified rat liver aminopeptidase B preparations were found to contain two molecular forms of the enzyme which can be separated by preparative electrophoresis on poly‐acrylamide columns. The one with higher electrophoretic mobility represents the actual liver enzyme and the one with lower electrophoretic mobility was seen to be derived most likely from hemolyzed red blood cells constantly present in liver homo‐genates. This was proved by purifying aminopeptidase B directly from isolated rat erythrocytes with the same procedure earlier used for the liver enzyme. The enzymic properties of the three enzymes (two derived from livers and one derived from erythrocytes) were seen to be largely the same. The similarity was especially considerable in the kinetics of the enzyme‐catalyzed reactions, where substrate inhibition was seen to be the most characteristic fe