Until recently, the hematopoietic stem cells (HSC) that appear early in ontogeny were thought to constitute a homogeneous, self-replenishing population whose developmental potential remains constant throughout the life of the animal. Studies reviewed here, however, demonstrated clear differences in the developmental potential of fetal and adult progenitor populations (including FACS-sorted HSC). These studies, which chart the ability of various progenitor sources to reconstitute functionally distinct B cell populations, define three B cell lineages: Bl a cells (CD5 B cells), derived from progenitors that are present in fetal omentum and fetal liver but are largely absent from adult bone marrow; B-lb cells (" sister" population), derived from progenitors that are present in fetal omentum, fetal liver, and also in adult bone marrow; and conventional B cells, whose progenitors are missing from fetal omentum but are found in fetal liver and adult bone marrow. B-la and Bl b cells share many properties, including self-replenishment and feedback regulation of development. These B cell studies, in conjunction with evidence for a similar develop mental switch for T cells and erythrocytes, suggest that evolution has created a" layered" immune system in which successive progenitors (HSC) reach predominance during development and give rise to differentiated cells (B, T, etc) responsible for progressively more complex immune func tions.