The mRNAs coding for the histocompatibility (H-2) antigens of the mouse have been identified by cell-free translation of poly(A)-containing RNA obtained from the livers of mice (strain A/J), followed by immunoprecipitation of the cell-free products by using an antiserum directed against purified H-2a. Unlike the 47,000- and 46,000-Mr H-2 glycoproteins synthesized in splenic lymphocytes, the cell-free translation products have Mrs of 45,000 and 44,500, representing the unglycosylated forms of these antigens. The cell-free products are shown to be related to the H-2 antigens by competition immunoprecipitation with purified H-2a and by two-dimensional tryptic peptide mapping. The H-2 mRNAs which sediment at 17 S are found associated predominantly with membrane-bound polysomes and are actively translated in the liver where as many as 16 ribosomes are associated with each molecule of H-2 mRNA. The implications of these studies for molecular cloning and for an understanding of the organization and expression of the genes encoding these H-2 antigens are discussed.