Polysomes released from microsomes of MOPC 41 mouse myeloma were used to prepare a poly(A)-containing fraction of RNA by chromatography on poly-(dT)-cellulose. From that fraction, a 14S RNA species was purified to a single peak by successive sucrose gradient centrifugations, followed by acrylamide gel electrophoresis. The RNA has an apparent molecular weight of 380,000 (1100 nucleotides), as estimated from the electrophoretic analyses. In a reticulocyte lysate this RNA directs the synthesis of a protein that migrates more slowly in sodium dodecylsulfate-acrylamide gels than does the light chain secreted by the same tumor. This difference in migration corresponds to a size difference appropriate for polypeptide chain about 20 amino acids longer than the light chain. The tryptic peptides of this protein correspond to those of the secreted light chain, except for the presence of two additional peptides from the product synthesized in vitro and for the absence of one light-chain peptide. The purified RNA is, therefore, the mRNA of the light chain, and it seems to code for a precursor protein slightly larger than the light chain. From the estimated size of the 14S mRNA, it appears that only 65% of the RNA is translated.