We have used W41/W41 (C57BL/6-Ly 5.1, Gpi-1b) anemic mice and a newly developed double congenic donor strain (C57BL/6-Ly 5.2, Gpi-1a) to determine if adult bone marrow (BM) injected in utero could provide stem cell engraftment. Of 38 fetuses injected intraperitoneally on day 13/14 of gestation with donor BM cells, 17 (47%) were live-born. On day 6, 12% had erythroid engraftment. On day 59, in 50% (8/16) of mice, 50% to 75% of erythroid cells, 42% of T cells, 5% of B cells, and 26% of granulocytes in the peripheral blood (PB) were derived from the in utero-injected donor BM. At 141 days, thymic, splenic, lymph node, BM, and PB chimerism studies showed that 57% to 80% of T cells, 10% to 15% of B cells, and 27% to 43% of granulocytes were of donor origin. At this time, BM was injected into irradiated secondary recipients. On day 104 posttransfer, a mean 23% of T cells, 8% of B cells, and 40% of granulocytes were derived from the in utero donor BM. These data indicate that adult BM has hierarchical engraftment capabilities in W41/W41 mice and prove that stem cells are engraftable in utero.