We describe here the construction and initial characterization of a 3-fold coverage genomic library of the human haploid genome that was prepared using the bacteriophage P1 cloning system. The cloned DNA inserts were produced by size fractionation of a Sau3AI partial digest of high molecular weight genomic DNA isolated from primary cells of human foreskin fibroblasts. former include an ability to clone DNA fragments that are up to 95 kb in size in an Escherichia coli host, an ability easily to use those clones to isolate and manipulate microgram quantities of recombinant DNA, an ability to produce libraries with a minimal effort (compared to YACs), and an ability to produce libraries that faithfully represent the genomic source DNA (9-12). In addition, the P1 cloning vector, pADl0sacBII, contains a positive selection for cloning and the vector is organized such that cloned inserts are flanked by rare-cutting restriction enzymes as well as by T7 and Sp6 promoters (4). To determine how well the P1 system is suited for genome mapping and sequencing it is important to produce and evaluate multicoverage P1 libraries. Toward this end a 3-fold coverage mouse library was constructed from cell line C127