Abstract: Secretin, which cannot be prepared directly by genetic engineering because of its carboxylic acid carboxyl-terminus, can be obtained by preparing secretylglycine by genetic engineering and then obtaining secretin therefrom by enzymatic conversion of the terminal glycine radical. The gene for the secretylglycine is synthesized chemically from smaller single-stranded units which are linked enzymatically to give the complete gene, incorporated into a suitable vector and amplified therein, after which the peptide is isolated directly or as a fusion protein and, after cyanogen bromide cleavage, is converted enzymatically into secretin.