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We obtained eight different cell lines in the long‐term bone marrow culture system that showed a germ‐line configuration of the joining (J) region segments of the Ig heavy‐chain (IgH) genes. Their surface markers were CD45R+, Ly‐1+, Lyb‐2+, cIgM‐, sIgM‐, Ia‐, Thy‐1‐, Mac‐1‐, and IL‐2R (Tac)+. Use of very young mice and the presence of IL‐5 were important for preferential promotion of the survival of B‐lineage lymphocytes bearing the Ly‐1 markers. When we treated two of them (J8 and J10) with 5‐azacytidine for 24 h followed by co‐culture with stromal cells and IL‐.5, they became Ly‐1+, sIgM+ B cells, and Ly‐1+, Mac‐1+ macrophagelike cells, respectively. After other early lymphoid lines (J1, J8, and J13) were maintained by co‐culture with ST2 and IL‐5 for more than a year, they showed a heterogeneous DNA rearrangement profile of the J region segment of the IgH gene, although only J13 rearranged the κ …