Determination of the responsiveness of four IL-7–dependent pre-B cell lines to other stromal cell-derived factors. The response of the cells to IL-7 +/+ and −/− stromal cells was compared in the presence of suboptimal concentrations of rIL-7. Ten thousand pre-B cells were cocultured with 10,000 FACS-purified stromal cells for 4 days. The pre-B cells were then harvested and enumerated. (A) The response of BC715. (B) The response of BC76. The response of BC715 represents one of three cell lines that did not respond to any other growth factor besides IL-7. This was concluded because the presence of the IL-7 −/− SC does not appear to affect the amount of proliferation. Similar results were obtained with the pre-B–cell lines BC77 and 2E8. The response of BC76 represents a cell line that responded to another growth factor in combination with IL-7 because, in the presence of rIL-7, the amount of proliferation is greater on the IL-7 −/− SC than without any stromal cells. (□) No SC; (▴) IL-7 +/+ SC; (•) IL-7 −/− SC.