Flow cytometric analysis of nonadherent cells generated in long-term cultures of BM stromal cells. Cells were doubly stained with PE-conjugated antimouse CD45/B220 and with FITC-conjugated antimouse IgM. Greater than 90% of cells are B220⁺ B cells, with 78% carrying the B220⁺/IgM⁻ phenotype (pre-B cells), and 22% expressing the B220⁺/IgM⁺ phenotype (mature B cells). Solid peaks indicate the staining with isotype-matched MoAb controls. The data are representative of the weekly analysis of BM cultures from 10 individual C/EBPβ+/+ and C/EBPβ−/− mice.