Little is known as to why a large number of human diseases are influenced by the major histocompatibility complex. In some cases, a direct involvement of the products of the polymorphic class I and class II, aas well as the less variable products of the class III, genes has been proposed. During characterization of the class III region for the presence of additional loci, we have located a duplicated locus encoding the major heat shock protein HSP70 between the complement and tumor necrosis factor genes. The HSP70 loci are 12 kilobases apart and lie 92 kilobases telomeric of the C2 gene. As HSP70 proteins have been linked with a protective role during and after cellular stress, and HSP70 analogues are often presented as antigens in bacterial and protozoal infections, this finding may have major implications with regard to the major histo-compatibility complex and associated diseases.