Abstract An investigation was made into the nature and extent of phosphorus and of nonhistone phosphorylated molecular species in histones and histone fractions. The distributions of radiophosphorus, of labeled nucleic acid precursors and of intact labeled nucleic acids during histone preparation and fractionation were presented. Evidence was obtained confirming the presence of O -phosphoserine in histone chromatographic fractions R, I and III-IV. Both RNA and DNA, or their derivatives, were shown to accompany certain histone fractions through extensive fractionation procedures. Further observations suggested that RNA, or a RNA derivative, was uniquely and firmly bound to certain histone subfractions. The significance of these observations was discussed.