The fetal mouse omentum has been shown to be a source of precursors that exclusively reconstitutes Lyl+ B cells and the closely related Lyl− sister population, but not conventional B cells or T cells. We have extended these studies to compare B cell development in the human fetal omentum, liver, and spleen, and to demonstrate that the pro/pre-B cell compartment (CD24+, sIgM−) is detected in the omentum and liver but not spleen as early as 8 wk of gestation. From 8 to 12 wk of gestation, the proportions of IgM+ cells that were pre-B cells (cIgM+ AIgM−) in the omentum and liver were 53 ± 15% and 45 ± 13%, respectively, and IgM+ cells were not detectable in the spleen. After 12 wk, the percentage of pre-B cells was unchanged in the fetal liver (41 ± 10%) but decreased significantly in the omentum (25 ± 14%); pre-B cells were now detected in the spleen but at much lower percentages (2 ± 3%) than either the omentum or liver. The nuclear enzyme, Tdt+, was detected in approximately 25% of the CD24+ cells in the omentum and liver during the 8-12-wk time period, however, Tdt + cells were not detected in the spleen. Approximately 40% of the mature B cells found in the omentum and spleen were CD5+ compared with only 20% in the liver. These results demonstrate that the fetal omentum, like the fetal liver and bone marrow, is a primary site of B cell development. © 1992, Rockefeller University Press., All rights reserved.