We studied the sequence repetitiveness of human cDNA and genomic DNA fragments inserted in the miniplasmid piVX. Sequence repetitiveness was assayed by the frequency with which a given insert mediated recombination between the chimeric miniplasmid and a recombinant bacteriophage library constructed from large random human genomic fragments. The methodology allows rapid analysis and isolation of sequences of a given copy number in the genome: few (1 to 10 copies), low order-repeated (10 to 100 copies) and a more highly repeated (over 100 copies). In a model application of the method, the distribution of these classes of sequences was compared in cDNA and genomic DNA libraries constructed in piVX. The major difference observed between cDNA and genomic DNA repeat structure was the paucity of highly repeated elements in cDNA copies from high-molecular-weight cytoplasmic poly(A) + RNA.