Three factors have been suggested to account for the fact that the mammalian foetus, which ordinarily differs from the mother in its genetic constitution, escapes the usual fate of homotransplants and continues to survive and develop in the mother throughout the period of gestation. These are (a) the anatomical separation of the foetus from the mother, (b) the antigenic immaturity of the foetus and (c) the immunological inertness of the mother. The present experiments were designed to assess the importance of the first factor. As a preliminary it was confirmed that the incidence of pregnancy in hybrid matings in rats and rabbits was unaffected by immunizing the female prior to mating with a massive skin homograft from the male. The fate of grafts of limb tissues transplanted from a foetus to its own mother 15 to 16 days after hybrid mating was then studied in both species. It was found that in normal mothers such grafts behaved like typical homografts in non-immunized recipients, whereas in mothers immunized by a skin graft from the male subsequently used for mating the foetal grafts encountered a violent reaction, histologically similar to the reaction which occurs with homografts of adult tissue in immunized hosts. These findings, it is suggested, point to the conclusion that the anatomical separation of mother and foetus is sufficient, and also necessary, to safeguard the mammalian foetus from the immunological hazards of pregnancy.