Bivariate Hoechstkhromomycin flow karyotypes for chromosomes from a Chinese hamster-human hybrid cell line (CH-Y-VII) were established that have retained a human Y chromosome. These bivariate flow karyotypes showed the human Y chromosome to be completely separated from the peaks for the Chinese hamster chromosomes. In preparative dual-beam sorting experiments, 3 x lo6 chromosomes were sorted from the Y peak into frozen petri dishes. An examination of Qbanded samples of sorted chromosomes revealed that 82% * 5% of them were human Y chromosomes.The DNA from the sorted chromosomes (approximately 250 ng) was isolated and used to establish a genomic library (vector X gt WESAB). Three clones (YACG 45, 52, 54) of Genomic libraries representative of the DNA of a specific chromosome (chromosome specific genomic library) may have a wide range of applications; e.g., for an analysis of the correlation of specific genetic traits with polymorphisms of the length of restriction fragments (4, 11,21,32), for establishment of DNA probes specific for particular chromosomes or chromosome regions (32,38), for investigations of tumor-related genetic occurrences (ll), or for studies concerning chromosome evolution (7,25). In principle, such investigations may also be performed using a library of the entire genome; however, the availability of chromosome-specific libraries should considerably facilitate them.The most direct way to construct a chromosome-specific library is to purify the chromosome of interest by fluorescence activated sorting (20) and to establish a genomic library from the DNA of the sorted chromosomes. This approach has recently been used to establish a genomic library for DNA from the X chromosomes this library containing inserts of repetitive human DNA were used for chromosomal localization by means of in situ hybridization to metaphase spreads of male human lymphocytes and of CH-Y-VII cells. In all three cases, a significant binding to the human Y chromosome was observed. A more detailed study of the chromosomal distribution of sequences homologous to the insert of YACG 45 suggested the existence of minor binding sites on several human autosomes. Southern blot analysis revealed the existence of other human specific sequences without Y chromosome specificity.Key terms: Isolated chromosomes, chromosome sorting, genomic libraries, in situ hybridization from man (12) and mouse (261, and from human chromosomes 21 and 22 (22), respectively.In this paper, we report on the preparative dual-beam sorting of the human Y chromosome for the construction of a Y chromosomal genomic library and a characterization of the Y specificity of cloned DNA probes. The human Y chromosomes (in this context also simply referred to as "Y chromosomes") were purified by dualbeam sorting (13,141 from a Chinese hamster-man hybrid cell line retaining the Y as the only free human chromosome (9).