A radioimmunoconjugate consisting of 3F8, a murine anti-GD2 ganglioside monoclonal antibody labeled with iodine 131 (I-131), with radioimaging and radioimmunotherapeutic properties. Using monoclonal antibody 3F8 as a carrier for I-131 results in the targeted imaging and/or destruction of cells expressing GD2. GD2 is a ganglioside which is overexpressed in malignant melanoma, neuroblastoma, and small cell carcinoma of the lung.