Abstract—1In vitroincorporation of tritiated leucine into nuclear proteins of normal human brain, astrocytoma I and II and glioblastoma multiforme was investigated. The distribution of radioactivity among various protein fractions of nuclei was determined.2The results demonstrate that the isolated nuclei of astrocytoma I and II incorporate radioactivity into proteins at least 40 times more actively than nuclei of normal human brain or glioblastoma multiforme.3The residual protein fraction was the most highly labelled fraction among the nuclear pioteins. This fraction from astrocytomas incorporates relatively more radioactivity than the similar fraction of normal human brain or glioblastoma multiforme. The buffered‐saline soluble protein fraction of astrocytoma nuclei contained a relatively lower amount of radioactivity than the similar fraction of normal human brain or glioblastoma multiforme. The total radioactivity incorporated into the deoxyribonucleoproteins seemed to increase with the malignancy of the tissue investigated. The significance of the results with respect to malignant transformation is discus