On the basis of previous studies supporting that glutathione (GSH) reduced cisplatin nephrotoxicity we have designed a new regimen in the treatment of advanced colorectal cancer, which included GSH as a modulator of cisplatin-induced toxicity. Eleven untreated patients with measurable metastatic colorectal cancer received 5-fluorouracil (750 mg/m2, daily continuous infusion for days 1-5) and cisplatin (40 mg/m2 1 hour-infusion for days 6-8) given every 4 weeks. Reduced glutathione (2.5 g) was delivered i.v. prior to each cisplatin infusion. Toxicity was minimal and reversible and included nausea/vomiting (11 cases), mild neurotoxicity (4 cases) and leukopenia (2 cases); only 2 patients showed moderate and transient increases of serum creatinine (less than 2 mg/dl) and BUN. Renal function impairment was also monitored by magnesemia levels and urinary marker enzymes indicating minimal cumulative nephrotoxicity. Out of 10 evaluable patients, only 2 partial responses were observed. The median survival was 9 months (range 5-26). The study was closed, since the preliminary results do not suggest any therapeutic advantage in adding cisplatin to 5-fluorouracil in the present schedule, even using an intensive regimen. Indirect evidence suggests that these disappointing results are not the consequence of interference of GSH on the cytotoxic efficacy of cisplatin. The lack of incidence of severe toxicity of this regimen supports the role of reduced glutathione as a potential protective against cisplatin nephrotoxicity. Although these preliminary results suggest that further studies with the present regimen in this disease are not warranted, in view of its safety this program deserves evaluation in the treatment of neoplastic diseases responsive to 5-fluorouracil/cisplatin.