Glutamine, which is expected to be produced by C6 glioma cells, is not detected in both amino-acid analyses and 13C-NMR spectra of perchloric acid extracts of cells incubated for 4 h with [1-13C]glucose in the absence of extracellular glutamine. However, the resonances of a glutamate-linked product are observed in these spectra. The analysis of the pH dependence of chemical shifts from various glutamate-derived compounds shows that the observed resonances came from glutathione. Glutamine and glutathione signals are in close proximity on the frequency scale, leading to possible misinterpretation of the spectra.