Iodine 131 (I131) tositumomab is composed of murine anti-CD20 antibody linked to the radioisotope I131. I131 tositumomab exploits the specificity of monoclonal antibody therapy and the radiosensitivity of non-Hodgkin's lymphoma to exact a combined antitumor effect. It is currently approved in the USA for treatment of relapsed or refractory low-grade (and transformed) non-Hodgkin's lymphoma. High response rates in this setting, with some patients achieving a remission of longer duration than that of their prior treatment, provided the basis for approval of the drug. Studies using I131 tositumomab as initial treatment for low-grade non-Hodgkin's lymphoma (with or without chemotherapy), and in conditioning regimens for autologous transplantation, have yielded promising results. Randomized, comparative trials are needed to define its optimal clinical use.