Human gene mapping would be greatly facilitated if marker loci with sufficient heterozygosity were generally available. As a source of such markers, we have used cosmids from a human genomic library. We have developed a rapid method for screening random cosmids to identify those that are homologous to genomic regions especially rich in restriction fragment length polymorphisms. This method allows whole cosmids to be used as probes against Southern transfers of genomic DNA; regions of cosmid probes homologous to repeated genomic sequences are rendered unable to hybridize with Southern transfers by prehybridization of the probes with a vast excess of nonradioactive genomic DNA. From one cosmid identified by this procedure, we have isolated three single-copy probes that collectively identify seven polymorphic loci. Of 56 unrelated individuals, 52 were heterozygous at one or more of these marker loci.