Therapeutic insulin preparation and a process for the production of a stable insulin preparation with protracted effect

Novel stable insulin preparations with protracted action and reduced or no antigenicity are obtained by reacting insulin free of antigenic impurities with an organic compound having basic amino groups, preferably a basic polypeptide or a cleavage product of a basic polypeptide, such as protamine, in a buffered protein dissociating or protein depolymerizing aqueous medium. Examples of preferred protein dissociating or protein depolymerizing media are aqueous solutions containing urea, a lower alkanol dialkylformamide, acetamide, N-alkyl-acetamide or acetonitrile.