Purified ovalbumin messenger RNA was employed to selectively enrich the concentration of the gene coding for ovalbumin from total chick DNA by molecular hybridization. The coding strand of the ovalbumin gene was partially purified from sheared chick DNA by affinity column chromatography using ovalbumin mRNA immobilized on phosphocellulose. The concentrations of the ovalbumin DNA sequence in various DNA fractions were quantitated by measuring their rates of hybridization with 125I-labeled ovalbumin mRNA. When apparent Cot1/2 values of these reactions were compared to the apparent Cot1/2 value obtained from the hybridization reaction between 125I-ovalbumin mRNA and complementary DNA synthesized against ovalbumin mRNA using the enzyme reverse transcriptase, purification of the coding ovalbumin DNA strand over total chick DNA was estimated to be approximately 9,600-fold. There was no apparent degradation of the 4,000 nucleotide strands of chick DNA throughout the purification procedure. Since ovalbumin mRNA has a complexity of 1890 nucleotides, the resulting DNA was more than twice the length of ovalbumin mRNA and thus should contain DNA sequences adjacent to the structural portion of the ovalbumin gene.