The cycling B precursor cells in rat bone marrow (BM) that carry the B220 antigen and no surface Ig daily produce 780 million new cells. The pool of recirculating B lymphocytes in the rat, however, renew at a rate of only about 40 million cells/day. To analyze at which stages in B lymphocyte genesis the cell loss occurs, we identified post‐mitotic cells in the rat BM B lineage, and determined their renewal rates. We used 5‐bromo‐deoxyuridine (BrdUrd) to label DNA‐synthesizing cells, identifying incorporated BrdUrd with the mouse monoclonal antibody BU‐1. B lineage cell subsets were identified by the markers HIS24 antigen (rat B220), terminal deoxynucleotidyl transferase (TdT), Ig μ heavy chain, and complete Ig. By use of double and triple immunocytology, we determined the extent of BrdUrd incorporation in the various B lineage compartments [HIS24+TdT−Ig−, TdT+, cytoplasmic μ chain (cμ)+ surface (s) IgM− pre‐B, sIgM+ B]. Both sIgM+ B lymphocytes and all B precursors with cell diameters < 11–12 μm were virtually devoid of DNA synthesis, as indicated by S‐phase indices below 2%. In contrast, S‐phase indices of large B precursors ranged between 43%–66%.