We have purified three low-abundance hepatic mRNAs to near homogeneity by polysome immunoadsorption. The mRNAs coding for the precursor of ornithine transcarbamoylase [carbamoylphosphate:L-ornithine carbamoyltransferase, EC 2.1.3.3], the precursor of the beta-subunit of propionyl-CoA carboxylase [propionyl-CoA:carbon dioxide ligase (ADP-forming), EC 6.4.1.3], and cystathionine beta-synthase [L-serine hydro-lyase (adding homocysteine), EC 4.2.1.22], representing approximately 0.20, 0.02, and 0.015% of total hepatic mRNA, respectively, were purified 450- to 6,300-fold. We used the following steps: interaction of rat liver polysomes with an IgG fraction of monospecific antisera raised against each polypeptide; immobilization of polysome-antibody complexes on a protein A-Sepharose column; removal of the bulk of polysomes by extensive washing; dissociation of ribosomal subunits and elution of specific mRNA with EDTA; and isolation of the eluted mRNA by chromatography on an oligo(dT)-cellulose column. It seems likely that this procedure will permit isolation of other low-abundance mRNAs and subsequent cloning of their respective cDNAs.