Inter-alpha-trypsin inhibitor (ITI), called inter-alpha-inhibitor, is a 220 kDa serine proteinase inhibitor found in human serum. It is composed of at least three distinct polypeptide chains. These chains, named H1, H2 and L, are an independently synthesized and proteolytically processed precursor protein. Only the complete structure of H2 and L has been established so far. We used a PCR-based cloning approach and a cDNA screening library to isolate the full-length cDNA H1. The amino acid sequences of the two heavy chains deduced from the cDNA are highly similar (40% identity). Nevertheless, the structure of the signal peptide and propeptide in the N-terminal region is different in these two chains. A complex posttranslational cleavage at both ends of H1 and H2 may be proposed prior to assembly of the ITI chains.