Fifty-six chromosome 18-specifιc yeast artificial chromosomes (YAC) were isolated from a human monochromosomal somatic cell hybrid DNA library. Fluorescence in situ hybridization revealed that the clones were evenly distributed throughout the chromosome. The clones, with an average insert size of 250 kb, cover about 18 % of the euchromatic part of chromosome 18. Of 90 STS markers tested, 17 were represented in this YAC collection. Together with our previously reported set of unique chromosome 18-specific YACs, we now have available 111 regionally mapped, essentially nonchimeric, clones that provide more than 30% coverage and form a framework for the complete physical map.