MPC-11 myeloma cells synthesizing IgG were labeled in vitro with 3H-uridine and the cell lysates, or purified polysomes treated with rabbit antiserum against the purified MPC-11 protein. The specificity of the immune precipitation was tested by rabbit anti-egg albumin and by precipitation of polysomes from “non-producer” myeloma and HeLa cells. The specific precipitation was 10 to 15% of the total cytoplasmic RNA for the IgG-producing clone of MPC-11 and about 2% for the non-producer clone or HeLa cells. Up to 20% of the total polysomal and ribosomal RNA were precipitated from the IgG producing clone. This value correlated with the IgG synthesis of these cells which is also about 20% of the total protein produced. The results suggest that myeloma cells produce a relatively large quantity of IgG due to the presence of a large amount of specific m-RNA molecules.