B-cell depletion in old hCD20 Tg mice reactivates B lymphopoiesis in the BM. Old (20 months) hCD20 Tg Balb/c mice (prescreened to have < 5% AA4.1⁺ B cells in peripheral blood as an indication for an “old-like” B-cell phenotype) were injected intraperitoneally with 1 mg/mouse of mouse antihuman CD20 monoclonal antibodies (clone 2H7). B-cell depletion was confirmed by staining of peripheral blood 5 days later. Mice were followed for 70 days until > 90% reconstitution of peripheral blood B cells was achieved. Mice were then analyzed by flow cytometry for the B lineage in the BM and compared with old (age-matched) and young (4 months old) hCD20 Balb/c controls). The BM cells were analyzed with a lymphocyte gate as defined by light scatter. Plots shown are representative of 4 mice in each group.