The presence of fetal cells, especially fetal nucleated red blood cells (NRBCs), in the blood of pregnant women is now a widely demonstrated phenomenon and is being considered as the basis for a novel noninvasive means for prenatal diagnosis. However, at present little is known about the fate of fetal cells once they enter the maternal circulation. Fetal cells could be removed from the maternal circulation by the maternal immune system or by the induction of apoptosis by other means (1)(2)(3). These proposals are supported by reports indicating the presence of terminal dUTP nuclear end labeling (TUNEL)-positive fetal NRBCs in the maternal circulation, a feature that may be attributable to the increased oxygen concentration in the maternal circulation (4), as well as by the description of apoptotic fetal cells or their remnants in the maternal plasma. Elimination of fetal cells by apoptosis from the maternal periphery may not be as widespread as suggested, or it may not affect all fetal cell types equally. Fetal leukocytes or hemopoietic progenitor cells may persist in the maternal circulation for years to decades after delivery (5)(6), and fetal cells with stem cell-like characteristics may even contribute to the regeneration of maternal tissue (7). Furthermore, microchimerism of the order seen in pregnancy frequently also occurs in solid organ transplant recipients and has, in these instances, been suggested to promote tolerance toward the graft (8). By analogy, it is therefore conceivable that the passage of fetal cells into the maternal periphery may promote tolerance against the semiallogeneic fetus. A caveat of previous studies on the status of fetal NRBCs in the maternal periphery is that they studied only nuclear events (TUNEL positivity) as indicators of the apoptotic status of trafficking fetal cells. Sekizawa et al. (2) reported that >40% the …