Immature and mature bone marrow plasma cell populations are disturbed in c-mpl^−/− mice. Mice were continuously fed with BrdU in the drinking water for 3 weeks. Subsequently, CD138⁺⁺/ intracellular κ light chain⁺ plasma cells were analyzed for BrdU content and quantified by flow cytometry. (A) Top panels: Flow cytometric identification of CD138⁺/Ig κ⁺ bone marrow plasma cells and analysis of BrdU incorporation. Bottom panels: Absolute numbers of BrdU⁺ and BrDU⁻ plasma cells in spleen and bone marrow of wild-type and c-mpl^−/− mice as indicated. (B) Ig serum levels of the IgG, IgA, and IgM isotypes in c-mpl^−/− and wild-type mice as measured by enzyme-linked immunosorbent assay. Data are mean ± SD for absolute plasma cell numbers of groups of 5 mice (statistics: unpaired t test).