Five G/C-containing oligonucleotides that include the recognition sequences of rare-cutting restriction enzymes have been used to isolate almost 100 different genomic segments from chromosome 7 that contain recognition sites for those enzymes. Hybridization and washing at 27 degrees C allow the use of 8-bp radiolabeled oligonucleotides to detect specific G/C-containing sequences in less than 1 ng of cloned DNA. This method was used to isolate 9 positive clones from 138 previously isolated single-copy probes from a flow-sorted chromosome 7 library. The specificity of the method was confirmed by showing that clones that gave positive hybridization signals also contained the corresponding restriction site. The oligonucleotides were also used to analyze approximately 12,000 kb of genomic sequence from a newly constructed chromosome 7 cosmid library that yielded 88 positive cosmids from 350 analyzed. The average distances between binding sites ranged from 200 to 690 kb and was independent of the number of CpG residues present in the oligonucleotide. Confirmation that clones containing restriction sites for these rare-cutting enzymes are located near genes was obtained by hybridization to RNA and cross-species DNA blots.