Complete, contiguous NotI restriction maps were constructed for the long arm of human chromosome 21 contained in nine different cell lines. These maps span about 43 Mb and consist of about 60 distinct NotI fragments, about 80 markers, and 11 chromosomal breakpoints. Although some differences among the maps were noted, the overall large-scale structure of the chromosomes is remarkably similar, which strongly validates the use of cell lines for mapping studies. Remarkably few serious discrepancies were found between the order of markers on this NotI restriction map and various genetic and physical maps. However, when compared with the NotI physical map, both the genetic map and the radiation hybrid map of chromosome 21 appear to expanded near the centromere and especially near the telomere.