This invention relates to a method of producing a modified (poly)peptide, said method comprising the step of modifying in an organic solvent a crown ether-bound (poly)peptide at one or more carboxylic groups by esterification or thioesterification and/or at the amino group of the N-terminal amino acid by amidation or alkylation. Furthermore provided are (poly)peptides and antibodies obtainable with the method of the invention as well as medical uses thereof.