Ribonucleoprotein particles were prepared by treatment of rat liver microsomes with detergents and high concentrations of KCl. They were active in incorporating 14C amino acids into protein when incubated with cell sap together with ATP, GTP, and a system to regenerate the triphosphates. The albumin of the incubation mixture, soluble at 105,000 g, and that of the fraction released by ultrasonication of the particles were studied by immunoelectrophoresis in agar gel. When the ribonucleoprotein particles were incubated with cell sap the immunological precipitation lines formed with antiserum to rat serum albumin were highly radioactive as tested by autoradiography. After zone electrophoresis on cellulose acetate, two immunologically reactive albumins were obtained which differed in their electrophoretic mobility from rat serum albumin. Labeled albumin, when purified on DEAE-cellulose columns, retained its radioactivity as tested by autoradiography following immunoelectrophoresis. On cellulose acetate this purified albumin showed an electrophoretic mobility higher than that of rat serum albumin.

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