In adult rats, as in other species, bone marrow has the highest potential for B-lymphocyte genesis as determined by a long-term repopulation assay for B-lymphocyte stem cells (1). These cells carry the Thy-1 and W3/13 antigens (2, 3). Pre-B cells characterized by the presence of µ heavy chains in the cytoplasm (cµ) but not on the cell surface (sµ) have been described in mice (4), men (5) and rabbits (6) but not in rats, and appear to be the immediate precursors of B-lymphocytes (7–10). The relation between pre-B cells and B-lymphocyte stem cells is not clear, however. Therefore, the aims of the present study were to identify cµ+sµ- pre-B cells in rat bone marrow and to find surface markers that would permit isolation of viable pre-B cells to subsequently assay their functional potential in vivo.