The biodistribution of 3F8, and IgG3 murine monoclonal antibody (MoAb) specific for the disialoganglioside GD2, was studied in nude mice xenografted with human neuroblastoma (NB). 3F8 conjugated to radioactive iodine and injected intravenously localized selectively to seven human NB when compared with Ewing's sarcoma and Hela cell xenograft controls. Uptake in NB was shown to be specific for MoAb 3F8 when contrasted with pooled mouse IgG or irrelevant IgG3 MoAb controls. Both small (50 mg) and large tumors greater than 2 g) showed radiolocalization. The percent injected dose uptake per gram tumor ranged from 8 to 50% and was inversely correlated with tumor size. Optimal tumor to normal tissue ratios were reached by 24-48 hr. There was no abnormal uptake in the reticuloendothelial system and the MoAb did not cross the blood-brain barrier. Based on the kinetics of the amount of radioactivity deposited in tissues, the relative radiation dose to normal organs was estimated to be 1% to 20% of the tumor dose. The MoAb 3F8 is useful for targeting radioactivity to human NB in vivo and the nude mice xenograft model may allow optimization of parameters that influence such biodistribution.