Neuroblastoma (NB) is the most common extracranial solid neoplasm of infancy and is associated with very poor prognosis in patients with advanced disease. Current therapeutic regimens of advanced NB which combine surgical resection with radiation therapy and/or chemotherapy brought some improvements, but in a significant number of patients, a cure remains elusive. Normal human serum of healthy adults contains natural IgM antibodies that are cytotoxic for human NB cells. In this study, we evaluated the anti- NB activity of these natural IgM antibodies in nude rats bearing solid human NB tumors. A single intravenous (i.v.) injection of purified cytotoxic IgM led to uptake of IgM into the tumors with massive perivascular complement activation and accumulation of neutrophil granulocytes after 24 hours. Five consecutive i.v. injections of purified cytotoxic IgM into NB-bearing animals resulted in complete growth arrest of even large and established solid tumors which lasted for several weeks after discontinuation of the injections, whereas tumors of control animals continued to grow exponentially during the observation period. These studies suggest that natural anti-NB IgM may have a potential as a novel therapeutic modality in the treatment of human NB.