Pre-alpha-inhibitor is a serum protein consisting of two polypeptides, the heavy chain and bikunin, covalently linked through an ester bond between the chondroitin sulfate chain of bikunin and the alpha-carboxyl group of the carboxyl-terminal residue of the heavy chain. The heavy chain is synthesized with a carboxyl-terminal extension, which is cleaved off just before the link to bikunin is formed. Our earlier studies indicate that this extension mediates the cleavage, and we have now found that a short segment on the amino-terminal side of the cleavage site is also required for the reaction. Furthermore, we previously showed that coexpression of the heavy chain precursor and bikunin in COS-1 cells leads to linkage, and we have now used this system to identify a His residue in the carboxyl-terminal extension that is specifically required for the intracellular coupling of the two proteins. In addition, we have shown that another chondroitin sulfate-containing protein, decorin, will also form a complex with the heavy chain, as will free chondroitin sulfate chains. These results suggest that in vivo there might be other, as yet unknown, chondroitin sulfate-containing polypeptides linked to the heavy chain.