The steady-state kinetics of the alkaline mesentericopeptidase-catalysed hydrolysis of esters of the general formula Ac-X-OMe(OEt) has been studied, "X" being an amino acid residue (Ala, Val, Leu, Ile, Phe, Tyr, Trp, Lys). The values of the specificity ratio kcat/Km indicate that the bonds involving the carboxyl group of amino acids with aromatic and bulky aliphatic side chain are hydrolysed most effectively. On account of this, alkaline mesentericopeptidase is classified as a chymotrypisn-like alkaline protease. The primary specificity of mesentericopeptidase reveals the similarity of this enzyme to the group of subtilisins, as well as the distinctive characteristic feature of the enzyme to hydrolyse Ac-Leu-OMe with an efficiency practically equal to that of aromatic amino acid derivatives. Suggestions are made about the nature of the substrate-binding centre, taking into consideration Schechter's and Berger's concept of the secondary specificity.