An optimal procedure for the isolation of ribonucleic acids (RNA) from whole cell and viral infected cell was developed. A considerable enzymic breakdown of RNA and the formation of jelly-like material could be circumvented by performing the extraction of pH: 9.5. DNA species were not observed in the preparations, indicating that the use of sodium naphthalene-1, 5-disulph-onate prevents the release of DNA into the aqueous phase.