We have established a murine in utero bone marrow transplantation model system and have investigated the effects of donor strain differences, cell dose, and the number of injections on murine fetal survival and en- graftment rates. In a series of experiments, 1221 non-anemic C57BL/6 fetuses were injected transplacentally on day 11 of gestation with 108 non-T-depleted adult bone marrow cells (BMC) from C57BL/6-CAST (con-genic), BALB/c and DBA/1 (allogeneic) strains without recipient conditioning. Overall fetal survival was 45%, with a 4% engraftment rate in 475 evaluable day 5 newborns. Engrafted newborns initially had up to 75-100% donor peripheral blood cell engraftment, partic- ularly with DBA/1 BMC. Surprisingly, a significantly (P<0.05) higher incidence of engraftment was observed using allogeneic (5.2%) as compared with congenic donors (0.7%). However, engraftment in all groups wastransient since engrafted recipients studied >6 weeks post-natally had nondetectable levels of donor cells. In contrast, engraftment of congeneic marrow into anemic, stem cell-defective Wv/Wv recipients lead to a higher incidence (40%) of engraftment that persisted for >6 weeks, increasing in the level of engraftment over time. Additional studies were performed in an attempt to further increase the incidence and permanence of engraftment. Neither doubling the cell dose nor doubling thenumber of injections improved engraftment rates in recipients of allogeneic bone marrow. Similarly, pretreatment of congeneic donors with 5-fluorouracil 4 days prior to harvesting marrow was not effective in increas-ing engraftment. Despite the inability to detect donor cells >6 weeks postallografting, 2 of 10 evaluable recipients engrafted with DBA/1 BMC had specific and permanent (>6 months observation time) tolerance to the donor skin graft with an intact capacity to reject third-party grafts. Thus short-term engraftment of allogeneic adult marrow stem cells can be successfully accomplished in a proportion of nonanemic fetal recipients. Engraftment of allogeneic donor cells can also lead to induction of a degree of tolerance in a proportion of recipients at maturity. These data form the basis of future studies directed toward understanding the mechanisms involved in in utero and postnatal marrow graft resistance, which will ultimately lead to designing strategies that will further enhance the permanence of engraftment with the use of adult marrow cells.