Preparation of milligram amounts of purified ovalbumin mRNA was accomplished by a sequential combination of precise sizing techniques with the selective purification of the poly(A) containing RNA by either affinity chromatography or adsorption to nitrocellulose filters. Several new techniques were applied to the purification of ovalbumin mRNA including Sepharose 4B chromatography and agarose gel electrophoresis in the presence of 6 M urea at pH 3.5. All the procedures used were adapted on a preparative sacle to the fractionation of large quantities of RNA. The purity of the ovalbumin mRNA was assessed by several independent criteria. (1) Purified ovalbumin mRNA migrated as a single band during both agarose-urea and formamide-polyacrylamide gel electrophoresis at pH 3.5 and 7.4, respectively. A single absorbance peak containing all of the ovalbumin mRNA activity was also found using linear formamide-sucrose gradients. (2) Determination of both total mRNA activity and ovalbumin mRNA activity in the wheat germ cell-free translation assay revealed that 92% of the total peptides synthesized were specifically immunoprecipitable with an ovalbumin antiserum. (3) Analysis of the total peptides synthesizied in the wheat germ assay by sodium dodecyl sulfate polyacrylamide gel electrophoresis demonstrated the presence of a single radioactive peak that corresponded exactly to a specifically immunoprecipitable ovalbumin standard. Thus, based on these observations ovalbumin mRNA appears to be greater than 95% pure. A preliminary estimation of the molecular weight of purified ovalbumin mRNA by formamide-containing sucrose gradients yielded a value of 520,000 or approximately 1600 nucleotides. This value was considerably less than the value of 900,000 obtained by gel electrophoresis under denaturing conditions. Analysis of the poly(A) content by a hybridization assay with (3H)poly(U) revealed the presence of a poly(A) region containing approximately 70 adenosine residues. Thus, the size of the ovalbumin mRNA is considerably greater than that required to code for a protein of 387 amino acids. The availability of large quantities of purified ovalbumin mRNA should now permit a more thorough analysis of its physical and chemical properties.