T200, a major cell-surface glycoprotein on lymphoid cells, exists in several forms with different electrophoretic mobilities. These forms have been correlated with different classes of lymphoid cell. The smaller forms, with molecular weights (MWs) of ≃170,000 and 180,000, are found predominantly on T cells while the 220,000 MW form is associated with B cells^1,2. The polypeptide portions of each molecule may be identical or closely related as all three forms share the same allelic variations^3–5, and all reported heterologous antisera and monoclonal antibodies to T200 precipitate all three forms^2,6. We report here a monoclonal antibody specific for the 220,000 MW form of T200 and show that it is expressed only on B cells and a subset of bone marrow cells which includes B cell precursors. We suggest that this form of the molecule be designated provisionally B220.