A murine IgG2 monoclonal antibody directed against the CD20 antigen, found on the surface of B-cells. Tositumomab binds to the CD20 surface membrane antigen, resulting in apoptosis, and may stimulate antitumoral cell-mediated and/or antibody-dependent cytotoxicity. OR A drug used together with another drug to treat certain types of B-cell non-Hodgkin lymphoma. It is also being studied in the treatment of other types of cancer. Tositumomab binds to a protein called CD20, which is found on the surface of B cells, and may kill cancer cells. It is a type of monoclonal antibody.