Absence of soluble inhibitory activities present in SCCM. SCCM from FACS-purified stromal cells does not inhibit the proliferation of (A) IL-7–dependent BC715 cells or (B) freshly isolated pro-B cells in the presence of 1.0 ng/mL rIL-7. In (B), FACS-purified pro-B cells (B220⁺CD43⁺IgM⁻) from young mice were used to maximize the detection of any factors present in the SCCM. The additional activity of the SCCM with IL-7 on freshly isolated pro-B cells is probably due to the presence of synergistic factors that are not active on the IL-7–dependent pre-B–cell lines. No inhibitory activities were found in SCCM with 0.1 to 5.0 ng/mL of IL-7, at 20% and 30% final volumes of SCCM, and on the other IL-7–dependent pre-B–cell lines (data not shown). At least 3 separate collections of SCCM have been tested with the freshly isolated pro-B cells and with each cell line.