Growth of human B-cell precursors (BCP) was achieved by plating fetal CD10+ surface-mu (s mu)- cells in liquid medium onto bone marrow- derived fibroblastic stromal cell layers deprived of hematopoietic cells. Proliferation of the fetal BCP was strongly potentiated by the addition of interleukin-7 (IL-7) to the cultures. Cultures included both a stroma-adherent and -nonadherent fraction of lymphoid cells, allowing us to expand the number of input BCP to 13-fold. In the presence of exogenous IL-7, proliferation was dose-dependent relative to the number of stromal cells, demonstrating that soluble IL-7 does not act alone to promote optimal growth. We further showed that the lymphoid cells recovered remain CD10+ sIg- BCP and that most cells expressed the maturation-associated CD20 antigen when IL-7 was added to the cultures. Whereas both freshly isolated CD20- and CD20bright BCP proliferated in the presence of stroma, we observed that high- proliferative capacity CD20dim cells were maintained in the cultures. Finally, CD20dim sorted cells were shown to subsequently acquire high levels of CD20 expression in culture, thus demonstrating a partial maturation sequence. The present culture system thus represents a useful model for studying the regulatory signals in early human B lymphopoiesis.