In recent years investigators have attempted to develop more rapid and precise methods to isolate specific chromosomal DNA regions. In this paper we demonstrate a modification of the method first developed by Goss and Harris for generation of irradiation hybrids. The gene encoding the dominant selectable marker for resistance to neomycin was introduced into human chromosome 4 using retroviral insertion into human fibroblasts. Transfer of these chromosomes via microcells into the mouse cell line NIH3T6 produced a somatic cell line containing chromosome 4 as the only human chromosome. Irradiation of this cell line followed by fusion with the hamster cell line CHTG49 generated hybrids containing only small portions of chromosome 4p on a hamster background. The use of selection produced stable hybrids that retained chromosome 4 fragments over long periods of tissue culture passage. To obtain new polymorphic markers for Huntington's disease, one of these hybrids was to isolate new genomic fragments. We identified 41 single-copy fragments, of which 27 have been mapped to specific regions of chromosome 4; 52% of these fragments map to the region of chromosome 4 containing the HD gene.