Radiolabeled monoclonal antibodies are efficacious in treating B-cell malignancies for the following reasons: B-lymphocytes, lymphoma, and myeloma cells are inherently sensitive to radiotherapy (Parker et al, 1980); the local emission of ionizing radiation by radiolabeled antibodies may kill cells with or without the target antigen in close proximity to the bound antibody; and penetrating radiation may obviate the problem of limited access in bulky or poorly vascularized tumors.