1. Mn2+-inhibited and Mn2+-activated aminopeptidases have been observed in ox brain and separated from one another by DEAE-cellulose column chromatography. 2. The Mn2+-inhibited enzyme has been purified 36-fold; it exhibits a specificity for tripeptide substrates, whereas the Mn2+-activated aminopeptidase cleaves dipeptides as well as tripeptides. 3. Ammonium sulphate treatment generates a Mn2+-stimulated aminopeptidase that is stable to dialysis against EDTA and water, in contrast with an endogenous Mn2+-activated preparation that is irreversibly denatured by such dialysis against EDTA and water.
