Abstract: Human insulin or B-30 esters thereof are prepared by reacting an insulin derivative of the formula ##STR1## wherein R is hydroxyl or an amino acid radical being different from threonine, and -A- and -B- represent the A- and B-chains with the same amino acid sequence as in human insulin, with a carboxyl protected and optionally hydroxyl protected L-threonine derivative in a concentration of from 2 to about 6 moles per liter in the presence of trypsin or a trypsin-like enzyme in a reaction medium containing water and optionally also an organic cosolvent at a pH value of from 5 to 9 and at a temperature below 50.degree. C., followed, if desired, by removal of any protecting groups present.Human insulin is prepared in an easy and simple way in high yield and in high purity.

Abstract: 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.