This chapter discusses the estimation of RNA by means of the orcinol reaction, which is a general reaction for pentoses and depends on the formation of furfural when the pentose is heated with concentrated hydrochloric acid. Orcinol reacts with the furfural in the presence of ferric chloride as a catalyst to give a green color. Only the purine nucleotides give any significant reaction. Mix two millilitre of the nucleic acid solution with three millilitre of orcinol reagent. Heat on a boiling water bath for twenty minutes, cool and determine the extinction at six hundred and sixty five nanometre against an orcinol blank.