To evaluate the size-fractionated cDNA libraries of human brain previously constructed (O. O'hara et al. DNA Research, 4, 53-59, 1997), the occurrence of chimeric clones and the content of clones with coding potentiality were analyzed using the randomly sampled clones with insert sizes of 5 to 7 kb. When the chromosomal location of 30 clones was determined by the radiation-hybrid mapping method, the map positions assigned from the 3'- and 5'-end sequences separately were coincident for 29 clones, suggesting that the occurrence of chimeric clones is at most 1/30. Using 91 clones mapped to chromosome 1, the content of clones that have the potentiality coding for proteins larger than 100 amino acid residues was estimated to be approximately 50% (46 out of 91 clones) on the basis of nucleotide sequence analysis and coding potentiality assay in vitro. No significant open reading frames were detected in the remaining clones. Although the clones coding for short peptides may not have been included in the above estimation, the libraries constructed from the whole brain mRNA fraction appear to contain a considerable amount of clones corresponding to the 5'-truncated transcripts in an unprocessed form and/or those with long 3'-untranslated regions.