Glutathione may not always be elevated in plasma in c-GT deÐciency, since it was not elevated in our patient, being <5 kmol/L. The Ðnding of a low-normal level of plasma c-glutamylglutamine, a stable metabolite (Hammond et al 1990), is Ðrm evidence that c-glutamyl compounds do not necessarily accumulate in blood in this disorder. Results from the experiment that compared the levels of compounds in the fresh urine with those in aliquots incubated at 37¡C for a further 4 h indicated three important points. (1) The urinary thiols, reduced glutathione, c-glutamylcysteine and cysteine, do participate in thiolÈthiol oxidation over time to form the disulphide compounds that were identiÐed in previous studies (Goodman et al 1971 ; Griffith and Meister 1980 ; Hammond et al 1995 ; OÏDaly 1968 ; Wright et al 1979). (2) Cystine is not elevated in the freshly passed urine. Griffith and Meister (1980) had considered that mild "cystinuriaÏ, and not cysteinuria, was a feature of this disorder. (3) No signiÐcant enzymatic conversion of reduced glutathione to c-glutamylcysteine occurred. Gross excretion of cysteine and c-glutamylglutamine, as well as reduced glutathione, has now been documented in fresh samples from our patient with c-GT deÐciency. None of these compounds appears in normal urine, and cglutamylglutamine is not excreted in hyperammonaemic patients even when the plasma level reaches 200 kmol/L (Hammond et al 1990). These are therefore the quantitatively important compounds that are normally salvaged from the glomerular Ðltrate by c-glutamyltransferase. As far as can be ascertained, this study contains the Ðrst in vivo evidence that cysteine is a physiological substrate of human renal c-glutamyltransferase. It is also the Ðrst report of cysteinuria from a metabolic cause.