A method is described for the estimation, by furfural formation, of small amounts of ribosenucleic acid (RNA) in the presence of large amounts of deoxyribosenucleic acid. Evidence is produced to show that the Schmidt & Thannhauser method does not give a satisfactory separation of the two types of nucleic acids. The method has been applied to the estimation of the RNA content of nuclei isolated from a number of different types of cell, and the values reported. It is concluded that the nuclei as isolated contain small amounts of RNA, but the possibility of adsorption of cytoplasmic RNA by the nuclei during the isolation procedure cannot be excluded. No correlation was found between the RNA content of the isolated nuclei and the prominence of their nucleolar systems.