Abstract Diazenedicarboxylic acid bis ( N, N-dimethylamide), diamide, completely oxidizes the glutathione of nucleated mammalian cells to the disulfide. The reaction is complete within seconds and is fully reversible. The stoichiometry for the reaction is 2 moles diamide: 1 mole cellular GSH, although the exact shape of the dose-response curve depends upon cell age and type, the temperature at which the reaction is performed, and the suspension medium used. Diamidetreated cells regenerate their original GSH content within 30–45 min at 37 ° or 23 °C in the presence of glucose; regeneration also occurs at 4 ° but it is slower. Diamide depresses DNA, RNA and protein synthesis, but these effects are completely reversed after the cells regenerate GSH (unless excess reagent is used) and cell division and subsequent growth are not impaired. An experiment is described which indicates that protein SH groups, but not GSH, may have a role in the progression of cells from mitosis into G1.