Abstract A RNA fraction characterized by a high rate of synthesis has been separated from the bulk of RNA of normal rat liver by a technique of RNA fractionation with phenol. This fraction was concentrated in the phenol-water interphase and extracted from it in an “undegraded” form by using bentonite throughout the procedure. Chromatography of this RNA on a methylated bovine serum albumin column showed that its main component resembled ribosomal RNA and that a rapidly labelled RNA component was eluted behind the main component. Further chromatography of the radioactive peak gave fractions differing considerably in the rate of isotopic labelling but with guanine plus cytosine contents consistently lower than that of the bulk of the cellular RNA. A fraction with the highest rate of labelling had the lowest guanine plus cytosine content.