cG250 is an IgG1 kappa light-chain chimeric monoclonal antibody that binds to a cell surface antigen found on 95% of clear-cell renal cancer. A multicentre phase II study was performed to evaluate the safety and efficacy of repeated doses of cG250. Thirty-six patients with metastatic RCC were included. All patients were nephrectomized for the primary tumour. Twenty-one patients were pretreated (e.g. with IL-2, IFN-alpha). A weekly dose of 50 mg cG250 was given by i.v. infusion for 12 weeks. Patients with SD or tumour response (PR, CR) after 12 weeks of treatment could receive additional treatment for 8 more weeks. None of the 36 enrolled patients had any cG250 grade III or IV toxicity. Only three patients had grade II toxicity possibly related to the study medication. ELISA testing gave no evidence for relevant amounts of HACA. Eleven patients presented with SD and ten were eligible for extension treatment. After the end of the study in the follow-up period, one patient demonstrated a CR in week 38 and another patient with SD showed a significant reduction of the overall tumour load in week 44. Six additional patients with progressive disease at study entry were stable for more than six months after the treatment start. The weekly schedule of i.v. cG250 in patients with metastatic RCC was safe, very well tolerated and non-immunogenic in a 12-week treatment regimen. cG250 showed anti-tumour activity.