This paper describes a method for the analysis of microsome fractions from pea seedling roots by electrophoresis on cellulose acetate membrane. The method separates the lipid membranous component, nucleoprotein and free RNA. The synthesis of RNA in the growing root has been studied by incubating the seedlings in radioactive phosphate, and analysing the isolated microsomal fractions by electrophoresis. A rapidly labelled nucleoprotein could be distinguished from the ribosomes, which remain unlabelled for nearly one hour. After longer incubation times, the ribosomes become labelled. Citrate buffer causes the dissociation of a trace of highly labelled RNA from the labelled ribosomes. It was found that the base composition of this RNA and of the RNA of the rapidly labelled nucleoprotein, differs from that of ribosomal or transfer RNA and is somewhat more like that of DNA.