The route by which maternal $\gamma $-globulin reaches the circulation of 19-and 20-day-old foetal rats has been investigated by surgical experiments on the foetuses in situ and by the use of antibodies as marked globulin. Antibodies from immune rat serum injected into the uterine lumen entered the exocoelomic and amniotic fluids of the foetus by way of the yolk-sac splanchnopleur and amnion. Antibodies from immune rat serum administered by mouth to the foetus were absorbed rapidly in the gut and appeared in the circulation. Antibodies also were absorbed by, and entered into the circulation of, the inverted yolk-sac splanchnopleur from immune rat serum to which it was exposed. They still reached the foetal circulation when entry by both these routes was precluded by keeping the mouth closed and removing the yolk-sac splanchnopleur. In these circumstances antibodies probably were absorbed from cavities in the entodermal sinuses of Duval and must have been transported by the allantoic circulation, but the possibility of passage from the maternal circulation directly across the placenta could not be excluded.