In this study, the activity of the glutathione related enzymes, namely glutathione S-transferase (GST), glutathione reductase (GSSG-R), Selenium-dependent and -independent glutathione peroxidase (GPX) of various TGC tumors (n = 18) obtained from untreated patients, was compared to that of the corresponding enzymes of normal testicular tissues (n = 5). The enzymes of all tumorous tissues except teratomas were significantly less active, than the corresponding enzymes of nontumorous tissues. The GST was in seminomas 4.3-20-, in embryonal carcinomas 47-, and in mixed tumors 13-47-fold less active than in the normal testes. The GST activity of teratomas was about half of that of the normal tissues. The Se-independent GPX, component of GST alfa class, comprised about 90 percent of the total GPX activity in normal testis; however it was absent or barely detectable in all TGC tumors except teratomas. The latter had about the same GPX activity as the tumor-free testicular tissues. Apart from the teratoma, the GSSG-R activity of all TGC tumors was also suppressed to about one third of that of the normal testis. The insufficient function of glutathione related enzymes of TGC tumors may contribute to their sensitivity against treatment. The poorer prognosis of teratomas, however, may be explained by the relatively higher activity of their detoxifying enzymes.