The gene coding for the inhibitor subunit of the human plasma protein complex inter-u-trypsin inhibitor has been cloned. The exon structure of the gene corresponds with the organization of the protein in two distinct inhibitor domains. The exons coding for the inhibitor domains each comprise the complete information for a typical Kunitz-type proteinase inhibitor structure. In contrast to bovine aprotinin, the mature inhibitor protein contains amino acid residues flanking the inhibitor domains on both sides. These amino acid residues are encoded by additional exons.