Guinea pig RNA was isolated from liver homogenates by phenol-chloroform-ethanol extraction and applied to a Sigmacell type 50 column in Tris (40 mM) buffer containing KCl (1.0 M) and MgCl2 (0.4 mM). Messenger RNA was eluted from the column, precipitated in ethanol and NaCl (1.0 M) washed in the same solution and redissolved in distilled water. The m-RNA was then added to a mixture containing rabbit reticulocyte polysomes (in some experiments pretreated with a micrococcal nuclease) an energy source, rat t-RNA, hemin, 3H-labeled leucine, 19 unlabeled amino acids and optimal concentrations of K+ and Mg++. Total protein synthesis by micrococcal nuclease-treated reticulocyte lysate was increased nearly four-fold (3700 cpm to 13,260 cpm) in the presence of m-RNA. Under these conditions, single-chain precursors of guinea pig albumin (m.w. > 68,000) and of C4 (m.w. ≥ 205,000) were synthesized, each accounting for less than 0.5% of the newly synthesized total protein.