A vector system for obtaining high yields of human calcitonin (hCT) in Escherichia coil was designed. Multiple copies of a synthetic gene encoding hCT-Leu, which is a substrate for C-terminal amidation by carboxypeptidase Y (CPaseY), were linked and fused to the lacZ gene. Each copy of the hCT-Leu gene was flanked by arginine codons to create sites for cleavage by clostoripain. The resultant multimeric hCT fusion protein upon treatment with clostoripain was converted to monomers of hCT precursor containing a carboxy-terminal arginine residue (hCT-Leu-Arg). The hCT-Leu-Arg was digested with carboxypeptidase B (CPaseB), amidated with CPaseY and thus converted to the mature hCT. In this way, we obtained the mature recombinant hCT.