Growing yeast cells incorporate radioactivity from labeled nucleotides, deoxynucleotides, deoxyadenosine, and deoxycytidine into acid-insoluble material. Most of these compounds are in corporated predominantly into RNA, only dTMP and dTTP serve as effective and selective DNA precursors. When yeast is prelabeled with dTMP, “specific” radioactivity of DNA decreases in the growing cells due to DNA augmentation. In X-irradiated yeast, “specific” radioactivity of DNA again decreases remarkably, though DNA augmentation of the growing giant cells in delayed. Simultaneously, radioactivity is rendered acid-soluble. Vice versa, X-irradiated unlabeled yeast incorporates radioactivity from labeled precursors into DNA, even if the DNA content of the cells does not increase. When growing yeast cells are treated with cycloheximide, protein synthesis and DNA augmen tation are inhibited completely with 5·10 -6 ᴍ cycloheximide. The decrease of “specific” radioactivity of DNA in X-irradiated cells, however, proves independent on an intact protein synthesis, even with 1·10 -4 ᴍ cycloheximide. The data provide clear evidence on DNA turnover in growing yeast after X-irradiation.