Carboplatin, a new platinum analogue, was administered intravenously on a schedule of a single dose every five weeks to 23 patients with advanced malignant solid tumors. Patients were treated at six dosage levels ranging from 200-550 mg/m2 every five weeks. Thrombocytopenia was dose-limiting. At 550 mg/m2 Carboplatin, the median platelet nadir was 65 000/mm3. Leukopenia was common, but usually of mild to moderate degree. Gastrointestinal upset was commonly seen at all dose levels, but 35% of the patients experienced no vomiting. No significant increase of the serum creatinine following Carboplatin was seen. In 16 patients serial determinations of the creatinine clearance were performed. The median base line creatinine clearance was 87 (50-155) ml/min and dropped to a median lowest creatinine clearance of 69 (23-171) ml/min on day four (p less than 0.05). The median creatinine clearance before the next Carboplatin treatment was 89 (42-155) ml/min. No significant proteinuria or electrolyte disturbances were noted. Carboplatin exhibited antitumor activity in ovarian, endometrial, thyroid and gastric carcinomas. The maximally tolerated dose appears to be 550 mg/m2 every five weeks. A starting dose of 450 mg/m2 seems to be appropriate for Phase II studies. In patients with impaired renal function and/or prior cis-Platin chemotherapy, Carboplatin at doses of 200-500 mg/m2 induced renal and severe hematological toxicity.