We have constructed a long-range physical map for 12 markers, including genes for GRIA4, IL1BC, and ACAT, across 9 Mb of chromosome 11q22-q23 in the region of the major locus for ataxia-telangiectasia (A-T). The markers fall into proximal and distal groups with respect to the centromere. We have linked the proximal and distal groups by hybridization to a 2.7-Mb NotI fragment and a 4.6-Mb MluI fragment. The following locus order was obtained: centromere-CJ52.75-J12.1C2-Y11B11R-IL1BC-hbcDNA-GRIA4-CJ52.3-Y11B29L-ACA T-CJ52.193-J12.8-Y11B06R-telomere. We show that hbcDNA/GRIA4 and CJ52.3 are very closely linked to each end, respectively, of the 2.7-Mb NotI fragment, thereby fixing the position of the complete contig. Our results indicate that the gene for A-T is flanked by the markers GRIA4 and J12.8, which are no more than 3 Mb apart, on a 4.6-Mb Mlu I fragment. The physical map allows rapid positioning of markers, and this will facilitate the construction of a YAC contig across the region.