PB and BM engraftment efficiency and early chimerism levels following in utero allogeneic and congenic BM transplantation. The number of mice that were injected with either congenic or allogeneic BM that demonstrated PB and BM chimerism at 1 week of age and PB chimerism at 1 and 6 months of age was assessed (Table 1). Levels of donor PB and BM chimerism were assessed in mice humanely killed at 1 week of age. No significant difference was noted in BM chimerism levels between congenic and allogeneic recipients. When allogeneic recipients with PB donor chimerism levels less than 1% were removed from the analysis, assuming that these are the mice in which chimerism is lost at the time of 1 month analysis, there was no significant difference in PB donor chimerism between congenic and allogeneic recipients (analysis on the right of the x-axis). Error bars represent the mean ± SEM.