1E10 is an anti-idiotype murine monoclonal antibody (Ab2 MAb) specific to an Ab1 MAb which reacts with NeuGc-containing gangliosides, sulfatides and with antigens expressed in some human tumors. Preparations containing this Ab2 were capable to induce a strong anti-metastatic effect in tumor-bearing mice. We conducted a Phase I clinical trial to evaluate the toxicity and humoral immune response elicited by 1E10 vaccine in patients with small cell lung cancer (SCLC). Eligible patients were those who after received chemotherapy and/or radiotherapy had partial or complete response to treatment. Patients received four biweekly injections with 2 mg of aluminum hydroxide-precipitated 1E10 MAb, then other six doses at 28-day intervals, and later the patients who maintained a good performance status were reimmunized. Six patients with limited-stage disease and three with extensive-stage disease were enrolled in the study. Most of the patients who received at least four doses of 1E10 vaccine developed strong specific antibody responses against 1E10 MAb and NeuGc-GM3 ganglioside. Antibodies able to react with lung carcinoma tissue sections were detected in sera from vaccinated patients. A prolonged survival was observed in several patients treated with the anti-idiotype vaccine. No evidence of serious adverse effects was found.