The inter-alpha-trypsin inhibitor (ITI) family is a group of structurally related plasma serine protease inhibitors. The ITI family members consist of combinations of mature heavy chains named HC1, HC2, HC3 linked to bikunin (a Kunitz-type protease inhibitor) by a covalent interchain protein-glycosaminoglycan-protein cross-link. The biosynthesis of the ITI family members takes place in the liver. In this report we examine the biosynthesis of these proteins using transient transfected COS-7 cells expressing one or more combinations of human ITI chains. The processing and secretion of alpha1-microglobulin and bikunin does not require the ITI heavy chains. A small proportion of the H3 chain seems to be processed into the HC3 form in the absence of the other ITI chains. In contrast, the processing of H2 into HC2 needs the presence of the L chain. The COS-7 cells are able to link the HC2 and HC3 heavy chains with bikunin by means of a chondroitin sulfate bridge, and thus to generate 260-kDa ITI-like proteins as well as pre-alpha-trypsin inhibitor (PalphaI). However, the maturation of the Hl chain into HC1 and the assembly of HC1 inside multichain proteins may take place according to a mechanism which differs from that of the H2 and H3 chains. These results indicate that the assembly of the constituent chains of the ITI-like proteins and PalphaI is not dependent on the liver machinery.