A direct hybridization protocol is described for screening cosmid and yeast artificial chromosome libraries with pools of Alu-PCR products from somatic cell or irradiation hybrids. This method eliminates purification, cloning and analysis of each individual Alu-PCR product before library screening. A series of human X chromosome irradiation hybrids were mapped by this method, using a cosmid reference library for comparisons between overlapping hybrids to identify interesting clones for further analysis.