We have developed a simple procedure based on reassociation kinetics that can reduce effectively the high variation in abundance among the clones of a cDNA library that represent individual mRNA species. For this normalization, we used as a model system a library of human infant brain cDNAs that were cloned directionally into a phagemid vector and, thus, could be easily converted into single-stranded circles. After controlled primer extension to synthesize a short complementary strand on each circular template, melting and reannealing of the partial duplexes at relatively low Cot, and hydroxyapatite column chromatography, unreassodated circles were recovered from the flow through fraction and electroporated into bacteria, to propagate a normalized library without a requirement for subcloning steps. An evaluation of the extent of normalization has indicated that, from an extreme range of abundance of 4 orders of magnitude in the original library, the frequency of occurrence of any done exmned in the normalized library was brought within the narrow range of only 1 order of magnitude. feasible task). Finally, by increasing the frequency of occurrence of rare cDNA clones while decreasing simultaneously the percentage of abundant cDNAs, normalization can expedite significantly the development of expressed sequence databases by random sequencing of cDNAs.Although cDNA library normalization could be achieved by saturation hybridization to genomic DNA (6), this approach is impractical, since it would be extremely difficult to provide saturating amounts of the rarer cDNA species to the hybridization reaction. The alternative is the use of reassociation kinetics: assuming that cDNA reannealing follows second-order kinetics, rarer species will anneal less rapidly and the remaining single-stranded fraction of cDNA will become progressively normalized during the course of the reaction (6-8). As we report here, we have used this kinetic principle to develop a method for normalization of a directionally cloned cDNA library that has significant advantages over two previously reported similar procedures (refs. 7 and 8; see Results and Discussion).