The invention relates to a process for the preparation of an insulin derivative of the formula I ##STR1## in which R.sup.1 denotes H or H-Phe, R.sup.30 represents the radical of a naturally occurring L-aminoacid and R.sup.31 represents a physiologically acceptable organic group of neutral or basic character consisting of 1 to 3 .alpha.-aminoacids in which the terminal carboxyl function is present in the free form and which insulin derivative has an isoelectric point above 5.8, which comprises reacting an insulin of the formula I in which R.sup.30 denotes the radical of a genetically codable L-aminoacid and R.sup.31 represents OH or a protective group of the carboxyl function, with a peptide or aminoacid derivatives of the formula R-R.sup.30 -R.sup.31 consisting of 2 to 4 .alpha.-aminocacids, in which the terminal carboxyl function is in free form, in the presence of a trypsin-like endopeptidase at a pH value below the isoelectric point of the starting insulin.