The restriction enzyme Pstl showed a two-allele restriction fragment length polymorphism (RFLP) marker when DNA polymorphism of the gene G8, a novel major histocompatibility complex (MHC) class III gene, was screened. The gene G8 is located ca. 4 kb centromeric of the heat-shock protein 70 (HSP70) gene cluster. A more detailed mapping indicated that the polymorphic restriction site is actually located in the coding region of the adjacent HSP70-2 gene, where it has been earlier reported to cause a silent mutation. To estimate the frequency of this polymorphism in the normal population, 95 blood donors were analyzed: The gene frequency of the 8.5 kb (designated 'L') allele was 0.45 and that of the 8.65 kb ('U') allele 0.55. However, when 19 families with patients suffering from celiac disease were studied, the gene frequencies in the affected haplotypes (L = 0.76, U = 0.24) significantly deviated from those observed in the normal population and in the non-affected MHC haplotypes of these families (L = 0.48, U = 0.52). However, the association results from a strong association between the allele 'L' and the MHC haplotype HLA B8 DR3, a known suspectibility marker of celiac disease. Only one patient, in fact, was negative for the well-established class II haplotype markers DR3 or DR7. The data therefore confirm the crucial role of MHC class II in suspectibility to celiac disease, but due to a strong linkage disequilibrium within MHC the role of MHC class III genes in disease associations can not be ruled out.