Background: Laboplatin, D-19466, 1,2-diaminomethylcyclobutaneplatinum(II) lactate, is a novel, water-soluble platinum complex which showed a better therapeutic index than Cisplatin or Carboplatin in murine and human tumor models. In a preceding Phase I study myelosuppression with predominant thrombocytopenia was the dose-limiting toxicity. Besides nausea and vomiting no further side effects were observed. Patients and Methods: The phase II study group of the AIO performed a phase II study in pretreated patients with advanced small-cell lung cancer (SCLC). 19 patients received Lobaplatin without hydration from which 18 were evaluable for toxicity and 17 for antitumor activity. The dose of 40 mg/m2 was given by intravenous bolus injection repeated every 4 weeks. Results: No objective complete or partial remissions were observed. 2 Patients showed a short-lasting no change. Thrombocytopenia was the dose-limiting toxicity: 11/18 (61%) patients presented WHO grades 1-4 and 5 WHO grades 3-4 (28%). The nadir occurred after 2 weeks with recovery within a week. Anemia WHO grades 1-3 were seen in 11 patients (61%), leukopenia grades 1-3 in 10 patients (56%). The nadir of leukopenia occurred after 3 weeks. Nausea and vomiting occurred in two thirds of the patients; however, were well controlled by antiemetics. No renal, CNS or ototoxi-city was observed. Conclusions: In patients heavily pretreated with chemotherapy, Lobaplatin did not show therapeutic activity at the dose of 40 mg/m2, although 33% of the patients experienced a grade 3 or 4 thrombocytopenia as the dose-limiting toxicity. Secondary resistance to chemotherapy and compromised bone marrow reserve due to intensive prior treatment may have contributed to this study outcome since Lobaplatin was clearly active in SCLC patients with no or one prior chemotherapy only, allowing administration of Lobaplatin at a dose of 50 mg/m2 and a repeat interval of 3 weeks.