AbstractThe relationship between surface IgM (sIgM) and surface IgD (sIgD) was examined on small lymphocytes in the adult murine bone marrow or prepubertal spleen. Cells were sorted on the basis of different sIgM levels by a fluorescence‐activated cell sorter (FACS) and relabeled for sIgD or total sIg by a sandwich technique using125I‐labeled protein A and radioautography. For detecting sIgD, an anti‐δ allotype reagent was used in congenic mice. Cells lacking sIgM in the bone marrow or spleen were also found to be sIgD−; thus, sIgD appeared only in the presence of sIgM. Weak sIgM‐bearing cells in the bone marrow also had no sIgD indicating that sIgD appeared only after the acquisition of a significant level of sIgM. Subsequently, the incidence of sIgD+cells increased in fractions showing increasing sIgM levels indicating the acquisition of new sIgD by “sIgM only” cells with increasing maturation levels in the bone marrow. In marrow lymphoid cells expressing both Ig isotypes, sIgM and sIgD levels increased in parallel, possibly with increasing maturation level. In the spleen, the incidence of sIgD+cells among various cell fractions showing different sIgM levels was found constant. However, spleen cells bearing both receptors, showed a small increase in the sIgD level with increas