The CD20 antigen has become a major therapeutic target in the management of follicular and other B cell non-Hodgkin's lymphomas. The murine monoclonal antibody, tositumomab, on binding CD20, is able to induce antibody-dependent cellular cytotoxicity, complement-dependent cytotoxicity and apoptosis. In addition, when radioiodinated, the antibody exploits the tumour's sensitivity to ionising radiation by direct targeting of the malignant cell. Tositumomab and Iodine (I(131)) tositumomab (Bexxar, GlaxoSmithKline, Philadelphia, PA, USA) is administered in two steps. The dosimetric step determines individual patient pharmacokinetics, allowing a patient- specific dose to be calculated. This is followed by the therapeutic step, with administration of the therapeutic dose between 7 and 14 days after the dosimetric dose. Over a decade's worth of experience in clinical trials has determined the efficacy and safety of the regimen in a variety of clinical circumstances; establishment of exactly where the regimen fits amongst the algorithm for the management of follicular lymphoma continues.