Passage of maternal cells into conceptuses in utero is recognized but poorly defined in species with hemochorial placentation. Despite the potential importance for such a phenomenon in vertical disease transmission, only limited data address the frequency of material to fetal cell trafficking or the developmental stage of its initiation. A murine model system, involving transfer of LacZ−, scid/scid, or wild type (+/+) blastocysts to pseudo-pregnant, LacZ+ transgenic ROSA26 females provided both flow cytometric and in situ information. In 100% of the late-gestation pregnancies studied, nucleated LacZ+ maternal cells crossed to conceptuses. In 90% of scid/scid fetuses, nucleated maternal cells were present in at least one lymphoid organ and often in more than one organ. Thymus was the most frequent site for maternal cell detection while the highest proportions of maternal cells were found in liver. Maternal cells were also visualized in fetal lung, heart, and bone marrow. Maternal cell trafficking into scid/scid fetuses commenced about midgestation, coincident with maturation of a placental circulation. In late-gestation +/+ fetuses, maternal cells were found extensively throughout bone marrow but not in other organs. The presence of maternal cells within primary lymphoid organs of fetuses may influence the repertoire of the developing fetal immune system and may be an underappreciated mechanism for vertical disease transmission.