The cisplatin derivative TNO-6 was evaluated for clinical toxicity in a phase I trial. TNO-6 was given daily for 5 days every 3 weeks as a 30-min IV infusion without hydration. In all, 39 patients with advanced cancer were treated at doses of 2.5-9.0 mg/m2. No dose-limiting nephrotoxicity occurred, but evidence of mild, reversible tubular damage was found. Dose-limiting toxicity was hematologic with both thrombopenia and leukocytopenia, which with high dose levels reached WHO grade 4. Hematologic toxicity was most pronounced for pretreated patients. No antitumor activity was seen. The recommended dose for phase II trials will be 9.0 mg/m2 for previously untreated and 8.0 mg/m2 for pretreated patients.