A DNA containing the coding sequence for the proteinase inhibitor protein hirudin from the leech 'Hirudo medicanalis' has been obtained by enzymic ligation of chemically synthesized deoxyoligonucleotides. The 226 bp synthetic gene carries signals for the translation initiation and termination. Fragment synthesis was performed by the Khorana ligation method as well as by the fill-in method. Efficiencies of these two methods are compared. The synthetic gene was expressed in 'E. coli' as a fusion protein with B-galactosidase under the control of the 'lac-promoter' as well as a non-hybrid protein under the control of the (symbol) PL-promoter. The non-hybrid expression product was shown to have similar biological properties as the authentic protein isolated from the leech.