The CD20 antigen (Bp35) is an integral non-glycosylated membrane protein with four transmembrane domains. Three isoforms (33, 35 and 37 kDa) result from differential phosphorylation within the cytoplasmic domain. The CD20 molecule is present on all normal B lymphocytes from peripheral blood, lymph node, spleen, tonsil and bone marrow but is absent from plasma cells. It is expressed naturally but at low density on a subset of peripheral blood T lymphocytes.