We describe here the construction of an ordered clone map of human chromosome 21, based on the identification of ordered sets of YAC clones covering > 90% of the chromosome, and their use to identify groups of cosmid clones (cosmid pockets) localised to subregions defined by the YAC clone map. This is to our knowledge the highest resolution map of one human chromosome to date, localising 530 YAC clones covering both arms of the chromosome, spanning > 36 Mbp, and localising more than 6300 cosmids to 145 intervals on both arms of the chromosome. The YAC contigs have been formed by hybridising a 6.1 equivalents chromosome 21 enriched YAC collection displayed on arrayed nylon membranes to a series of 115 DNA markers and Alu-PCR products from YACs. Forty eight mega-YACs from the previously published CEPH-Genethon map of sequence tagged sites (STS) have also been included in the contig building experiments. A YAC tiling path was then size-measured and confirmed by gel-fingerprinting. A minimal tiling path of 70 YACs were then used as probes against the 7.5 genome equivalents flow sorted chromosome 21 cosmid library in order to identify the lists of cosmids mapping to alternating shared--non-shared intervals between overlapping YACs ('cosmid pockets'). For approximately 1/5 of the minimal tiling path of YACs, locations and non-chimaerism have been confirmed by fluorescence in situ hybridisation (FISH), and approximately 1/5 of all cosmid pocket assignments have independent, confirmatory marker hybridizations in the ICRF cosmid reference library system. We also demonstrate that 'pockets' contain overlapping sets of cosmids (cosmid contigs). In addition to being an important logical intermediate step between the YAC maps published so far and a future map of completely ordered cosmids, this map provides immediately available low-complexity cosmid material for high resolution FISH mapping of chromosomal aberrations on interphase nuclei, and for rapid positional isolation of transcripts in the highly resolved regions of genetic interest.