NF-κB is a pleiotropic transcription factor that may be involved in regulating hematopoiesis. Mice deficient in both the p50 and p65 subunits of NF-κB die on day 12.5 of fetal development. To study hematopoiesis in the absence of these subunits, we produced radiation chimeras by introducing p50/p65 deficient day 12 fetal liver cells into lethally irradiated hosts. One month after transplantation, lymphocytes derived from p50/p65 deficient fetal liver cells could not be detected in the blood, spleen, or bone marrow of host animals, despite a marked overabundance of fetal liver derived mature and immature granulocytes. Animals injected with control fetal liver cells did not display these hematologic abnormalities. Although we considered the possibility that the absence of NF-κB caused an alteration in hematopoietic lineage commitment, this did not appear to be the case because immunoglobulin heavy chain gene D-J joints were detected in p50/p65 deficient day 12 fetal liver cells, and within this cell population, myeloid colony forming units were present in normal mumbers. Surprisingly, the hematologic abnormalities noted in host animals injected with p50/p65 deficient fetal liver cells did not occur if fetal liver cells were cotransplanted with wild-type bone marrow cells. These results suggest that NF-κB influences the development of hematopoietic precursors through the regulation of an extracellular factor.