Two messenger specific factors, phosphocellulose fractions 3 and 4, have been isolated from the initiation factor 3 fraction of red muscle initiation factors by chromatography on phosphocellulose. When added to a reticulocyte cell-free system containing both myoglobin and myosin mRNAs, phosphocellulose fraction 3 is found to specifically stimulate the synthesis of myoglobin while phosphocellulose fraction 4 is found to specifically stimulate the synthesis of myosin. In addition, a new RNA, isolated from the initiation factor 3 fraction, is shown to specifically inhibit the translation of heterologous mRNAs.