Because of their large size, YACs provide is a powerful tool for physical mapping studies of complex genomes. As it will be advantageous to have genomic libraries of clones with large inserts for analyzing megabase sized regions of the human genome, we have investigated a number of parameters in order to increase the insert size of the YACs. We constructed a genomic library currently containing more than 85,000 YAC clones. Mean sizes of YACs produced at several stages of construction of the library range from 430 kb to 1,200 kb, representing 13 haploid equivalents of the human genome. This library was organized in order to allow rapid screen of YACs for large scale physical mapping of the human genome.