AbstractTo gain insight into the function of cell surface IgM (sIgM) and IgD (sIgD), the expression of these markers on B lymphocytes was quantitated during activation and progression through the cell cycle. Specifically, analysis and correlation of changes in cell cycle state, sIgM and sIgD expression and cell size following exposure of murine B cells to mitogenic levels of lipopolysaccharide and dextran sulfate is reported. As assessed by flow cytometric acridine orange analysis, a large proportion of normal splenic B cells respond within 48 h of exposure to these mitogens by entry into the cell cycle. This response is accompanied by an increase in cell size as determined by flow cytometric “time of flight” measurement. Flow cytometric immunofluorescence analysis reveals a simultaneous alteration in sIg expression. Specifically, cells leaving G0and transiting G1increase in diameter from 5 μm to 6 μm and lose>80% of sIgD while sIgM remains constant. Progression through the remainder of the cell cycle is accompanied by a further increase in mean cell diameter to approximately 12 μm while sIgM and sIgD levels remain at G1levels. The abrupt loss of sIgD as cells transit G1suggests that an active process mediates this de