The cDNA for interleukin-7 (IL-7) was recently isolated from a stromal cell line derived from a long-term B-lymphoid culture. We report that purified recombinant murine IL-7 can promote the clonal growth in semi-solid culture of a subpopulation of cells expressing the B220 surface antigen from normal murine bone marrow. These colony-forming cells (CFC-Pre-B) give rise to colonies of 20 to 1,000 cells after 7 days in culture. Morphologic examination of cells within the colonies showed a characteristic lymphoid morphology, and histochemical examination demonstrated an absence of markers associated with granulocyte, macrophage, eosinophil, or megakaryocyte differentiation, as well as an absence of hemoglobinization (indicative or erythroid differentiation). IL-7 was found to specifically enhance the infection of CFC-Pre-B but not CFU-GM when the cytokine was present during a 48-hour co-cultivation period between irradiated, retrovirus-producing psi 2 clones and normal mouse bone marrow cells. In contrast, IL-3 enhanced the infection of CFU-GM but not CFC-Pre-B. Thymidine suiciding studies suggest that this targeted infection is due to specific induction of cycling of CFC-Pre-B by IL-7 and CFU-GM by IL-3. These data demonstrate that IL-7 can target retroviral infection into a specific subpopulation of early B-lymphoid cells (CFC-Pre-B), and that IL-7 cannot directly promote the in vitro clonal growth of myeloid committed progenitor cells (ie, CFU-GM).