Proteins HC and HI-30 of inter-alpha-trypsin inhibitor light chain are two plasma proteins. They are encoded by the same monocistronic mRNA. Their function is probably related to the regulation of immunologic and/or inflammatory responses. Using a genomic DNA probe and a panel of somatic cell hybrids we have mapped the gene coding for the proteins to chromosome 9. In situ hybridization experiments refined the assignment to the region 9q22.3----q33.