Abstract The non-protein sulfhydryl (NPSH) content of Ehrlich ascites tumor cells was measured in relation to cytokinetic parameters of tumor growth. Thymidine labeling index, NPSH, mitotic index, and protein were assayed under a variety of experimental conditions, both in the hours following transplantation and during subsequent daily growth of the tumor. In some experiments, X-radiation was used to block cells in G2. Cellular NPSH declines during progressive tumor growth. This decrease appears to reflect the increasing proportion of non-proliferating (G0 or G1-like) cells or the decreasing proportion of proliferating cells in the tumor. When cells from 7-day tumors are transplanted into fresh mice, G0 cells move back into the cycle at S and the mean cellular NPSH content more than doubles within 9 h. When large inocula are used, the post-transplantation increases in the proportion of cells synthesizing DNA and in the mean NPSH level are greatly reduced. These results suggest that G0 cells contain less NPSH than do proliferating cells. There is no evidence that proliferating cells contain more NPSH in late G2 or in mitosis than in any other phase of the cell cycle. The rise in cellular NPSH concentration following transplantation was not affected by prior X-radiation (1250 R), indicating that movement of G0 cells back into the cell cycle is not particularly radiosensitive.