The antitumor activity of zeniplatin, a third-generation, water-soluble platinum compound that has shown broad preclinical antitumor activity and no significant nephrotoxicity in phase I trials, was tested in patients with advanced malignant melanoma and advanced renal cancer. Patients who had not previously been treated, except with local limb perfusion and immunotherapy, were given zeniplatin as bolus injections at 125 mg/m2 every 3 weeks. The main hematological toxicity was leukopenia (7/30 patients, WHO grade > or = 3) and the main nonhematological toxicity was nausea and vomiting (21/30 patients, WHO grade > or = 2). Serious nephrotoxicity was observed early in the renal cancer study and, later, also in the melanoma study. Hyperhydration did not prevent the nephrotoxicity, and the studies were stopped after 6 renal cancer patients and 24 malignant melanoma patients had been included. Zeniplatin gave objective responses in 3 of the 21 evaluable malignant melanoma patients [2 complete responses (CRs) in patients with lymph-node metastases lasted 5 and 14 months, respectively; 1 partial response (PR) in a patient with lymph-node and liver metastases lasted 6 months]. In the renal cancer study, only four patients were evaluable for response and none responded. The results show that zeniplatin has some activity (14%) in patients with advanced malignant melanoma, but no conclusion can be drawn regarding the activity of zeniplatin in renal cancer as the number of patients was too low. The main toxicities were leukopenia and nausea and vomiting. Unexpected and serious nephrotoxicity was observed, and for this reason the studies were terminated before the planned number of patients had been included. A possible explanation for the nephrotoxicity may be drug interactions, but no firm conclusion can yet be drawn.