The preparation of isotopically labeled nucleic acid was undertaken to provide material for a study of nucleic acid metabolism and a source of labeled nucleotides and nucleosides, which are otherwise unavailable. For this purpose yeast was propagated in a manner found to produce a rich source of nucleic acid (1) on a medium supplying excess N15 in the form of ammonium sulfate. Nucleic acid was isolated by adapting the method of Redfern (2) to laboratory usage. From the nucleic acid were isolated adenine picrate, guanine sulfate, and silver pyrimidines. The isolated purines and pyrimidines contained essentially identical amounts of isotopic nitrogen (Table I). This level was slightly lower than the N15 content of the nucleic acid sample and was considered indicative of contamination of the nucleic acid sample by some proteinaceous material. The residual yeast (from which nucleic acid was extracted) showed a higher N16 content than did the nucleic acid. This could be attributed to the fact that there was proportionately more protein produced during the growth period, since the seed yeast contained 7.22 per cent nucleic acid (dry basis), whereas the final yeast contained only 6.37 per cent nucleic acid (dry basis). The molasses employed for the yeast propagation contained 20.93 gm. of nitrogen. It was calculated from the isotope analyses that 3.99 gm. (19.0 per cent) of this nitrogen was assimilated by the yeast.