We have used a kinetic approach to construct cDNA libraries containing approximately equal representations of all sequences in a preparation of poly(A)+ RNA. Randomly primed cDNA fragments of a selected size range were cloned in lambda phage vector. Inserts were amplified by the polymerase chain reaction (PCR), denatured, and self-annealed under optimized conditions. After extensive but incomplete reannealing, the single-stranded fraction was relatively depleted of more abundant species of cDNA. Libraries of these fragments are suitable for cDNA subtraction, screening, or selection by hybridization and make it possible to detect and analyze cDNA corresponding to species of mRNA present at a low level in a small fraction of the cells in a complex tissue.