Antineuroblastoma monoclonal antibody (MoAb) was labeled with iodine-131 (131I) and injected into transplantable human neuroblastoma-bearing nude mice in vivo. Imaging of the tumor by gamma camera was then attempted. At the fourth day after injection of the antibody, relatively heavy labeling was observed in the tumor. Principal organs were removed and their 131I uptake was determined. A high radioactivity count was detected in the tumor, indicating efficacy of this antibody in tumor imaging of neuroblastoma. This antibody was labeled with 131I and injected into transplantable human neuroblastoma-bearing mice. Apparent inhibition of tumor growth was observed in animals treated as such compared with animals of the control group, MoAb alone group, and 131I alone group (P less than 0.05). In addition, pathohistologic study showed binding of 131I to tumor cells and necrotic changes, suggesting the possibility of application of this MoAb to the treatment of neuroblastoma.