There were early results that effect of single intravenous transplantation of pregnant rats Percoll derived mononuclear bone marrow cells (BMC) suspention of 4-5, 7-9 or 11-12 pregnant days to rats with the same date of pregnancy depends on gestation period of transplantation. The weight of 18th day fetuses after intravenous transplantation during 7-9 days was increased with significant difference in comparison with weight of normal fetuses. In case of intravenous BMC transplantation during placentation at 11-12 days of pregnancy the weight of fetuses was decreased, the weight of placentas was increased and survival of fetuses was disturbed. There are several explanations of increase of weight of fetuses after intravenous BMC transplantation within gastrulation including paracrine effect of allogenic transplanted cells on the DC development, the apoptosis and proliferation of embryonal cells and possible epigenetic action. The retardation of fetuses growth after intravenous BMC transplantation during placenta formation at pregnant 11-12 days may be explained by cytotoxic action of uNK cells for embryo. At the same time the sub-dermal BMC transplantation at 11 and 13 pregnant days stopped the embryotoxic action of uNK cells and increased the weight of fetuses and preserved the survival of embryos.