Interleukin-7 plays important roles in the B cell developmental pathway including events leading to commitment, survival, proliferation, and maturation. Because of its central role in adult murine B lymphopoiesis, IL-7 is frequently used to generate B cell progenitors in vitro. We have shown that differentiation of IL-7-responsive cells in these cultures is influenced by CD45, pre-B cell receptor, and other downstream signaling molecules. A common, but often overlooked aspect of IL-7 containing cultures is the routine maturation of cells to the sIgM(+) stage. The production of B cells in IL-7 containing cultures is balanced by cell death, since such cells fail to survive for long without additional stimulation.