Tritiated thymidine (3-H-T) was administered intravenously to seven patients with cerebral gliomas. Autoradiographs of biopsy specimens excised within the next four hours and of autopsy specimens from three of these patients obtained three weeks to six months after the single pulse of 3-H-T revealed the following: (a) no gemistocytic astrocytes and only a few giant astrocytes were labeled in biopsy specimens, despite a high overall labeling index of 5-10% (percentage of cells labeled in the total cell population); and (b) scattered foci of labeled genistocytes occurred in autopsy specimens, despite a sharp drop in overall labeling index. In histologic sections of these same specimens, genistocytes and giant cells occurred as the major cell types in an irradiated tumor, in large clusters near foci of degeneration, and as isolated cells in anaplastic foci. These findings suggest that: 1) gemistocytes and giant astrocytes are similar in origin and growth potential regardless of minor variations in morphology; 2) these cells multiply slowly, if at all, and are closely related to regressive changes within the tumor; 3) these cells may reflect profound proliferative activity in adjacent neoplastic cells; and 4) the labeling index and malignant potential of the tumor as a whole depend upon the more rapidly dividing tumor elements. Thus, if genistocytes and giant cells indicate malignancy, they do so secondarily, and the biologically harmless gemistocyte may be the loser in an intense competition for the substrates needed in cell proliferation.