Preleukemic Eμ-myc/bcl-2^−/− reconstituted mice have abnormally low numbers of mature B cells. Spleen and LNs were harvested from lethally irradiated wt mice 8 to 12 weeks after reconstitution with fetal liver cells from wt, bcl-2^−/−, Eμ-myc, and Eμ-myc/bcl-2^−/− (E14.5) embryos. Single-cell suspensions were prepared, stained with fluorochrome-conjugated monoclonal antibodies to B220, IgM, IgD, and Ly5.2, and analyzed by FACS. (A) Representative FACS profiles illustrating the abundance of immature (R1: B220⁺ sIgM^hi sIgD^lo) and mature (R2: B220⁺ sIgM^lo sIgD^hi) splenic B cells, and total numbers in spleens from wt, bcl-2^−/−, Eμ-myc/bcl-2^+/+, and Eμ-myc/bcl-2^−/− fetal liver reconstituted mice. Profiles are gated on Ly5.2⁺ B220⁺ cells. (B) Total numbers of donor-derived mature B cells (B220⁺ sIgM^lo sIgD^hi) in LNs from wt, bcl-2^−/−, Eμ-myc, and Eμ-myc/bcl-2^−/− reconstituted mice. Graphs represent means ± SEM from 5 to 8 mice of each genotype. Statistically significant differences: *P < .05; **P < .001.