Inhibition of apoptosis in tumour cells may depend on intracellular reduced glutathione (GSH) level. In this work, GSH levels were studied by (1)H MRS in MCF-7 and HeLa cells, characterised by a different radiosensitivity. Annexin-V test showed that the fraction of apoptotic HeLa cells after irradiation is much higher than in control, although MCF-7 cells did not show a significant apoptosis. MRS signals from GSH (G) show lower intensity in HeLa with respect to MCF-7 cells; the opposite is true for free glutamic acid [glu (g)]. After irradiation, the G/g ratio decreases in MCF-7, although remaining approximately constant in HeLa cells. Buthionine sulfoximine (BSO) treated MCF-7 cells show an increase in the percentage of apoptotic cells; in parallel, G/g ratio behaves as in HeLa. This study indicates that GSH level may act as predictive marker of apoptosis by irradiation.