Extraordinary progress has been made in charting the maturation of hematopoietic cells. However, these charted processes do not necessarily represent obligate pathways to specialized types of lymphocytes. In fact, there is a degree of plasticity associated with primitive progenitors. Moreover, all lymphocytes of a given kind are not necessarily produced through precisely the same sequence of events. Particularly contentious is the nature of cells that seed the thymus, because different progenitors can generate T cells under experimental circumstances. Non-renewing progenitors with a high density of c-Kit in bone marrow are likely to replenish the thymus under normal circumstances and most closely resemble canonical T cell progenitors. © 2005 Elsevier Ltd. All rights reserved.