Biochemical features of renin have been studied. Determination of the amino acid sequence and catalytically essential groups in the active sites of mouse submandibular gland revealed the similarity of renin with acid proteases. Yet stringent substrate specificity, neutral pH optimum of its enzyme activity and the unique structure of the activation peptide distinguish it from digestive enzymes. Inactive renin was identified as renin zymogen by complete purification and translation in vitro.