Pre-alpha-trypsin inhibitor (P alpha I) is a serine-proteinase inhibitor of M(r) 130,000 found in human serum. This protein belongs to the family of proteins called inter-alpha-trypsin inhibitor (ITI). P alpha I is composed of a heavy chain (HC3) and of a light chain (bikunin), synthesized by two separate mRNA. Bikunin is identical to the ITI light chain, the structure of which has already been established. The HC3 is obtained from a precursor called H3. The bikunin is covalently linked to HC3 by a chondroitin-4-sulfate glycosaminoglycan. We report here the H3 full-length cDNA sequence and the deduced amino-acid sequence of the heavy-chain H3 precursor. The high degree of similarity between the nucleotide and amino-acid sequences of ITI heavy-chain families H1, H2, H3 is examined with respect to their probable structure and assembly with bikunin in the final proteins, P alpha I and ITI.