Tositumomab is a murine IgG2a lambda monoclonal antibody directed against the CD20 antigen, which is found on the surface of normal and malignant B lymphocytes. Tositumomab is produced in an antibiotic-free culture of mammalian cells and is composed of two murine gamma 2a heavy chains of 451 amino acids each and two lambda light chains of 220 amino acids each. The approximate molecular weight of Tositumomab is 150 kD. In vitro studies have demonstrated that upon binding to the CD20 antigen, Tositumomab is capable of inducing apoptosis. In addition, Tositumomab induces antibody-dependent cellular cytotoxicity (ADCC) and complement-dependent cellular cytotoxicity (CDC). Iodine I 131 Tositumomab is a radio-iodinated derivative of Tositumomab that has been covalently linked to Iodine-131. Unbound radio-Iodine and other reactants have been removed by chromatographic purification steps. This agent produces its cytotoxic effect through the antitumor effects of ionizing radiation, as well as the more direct antitumor effects of the antibody. The goal of treatment with Iodine I 131 Tositumomab is selective delivery of radiotherapy to radiosensitive malignant cells, thus minimizing toxicity to the normal organs.