Justification of cytoplasmic FACS as a method to compare the amount of IL-7 within the cytoplasm of stromal cells. (A) Comparison of the ability of 2 stromal cell clones to support IL-7–mediated proliferation of BC715 pre-B cells. (B) Histograms showing the levels of cytoplasmic IL-7 staining of the same 2 stromal cell clones as determined by flow cytometry. (C) Histograms comparing the levels of cytoplasmic staining of RS2 cells using normal Ig and anti–IL-7. (D) Net specific IL-7 staining for the same 2 stromal cell clones calculated as the difference in MFIs between staining with anti–IL-7 and control Ig antibodies. (▪), RS2; (▩), RS4.