1. According to the author's data by means of precipitin reactions the fetal blood sera contained a specific protein fraction which did not exist in the adult blood sera, and the same fraction existed also in placenta, amnion water and in the blood sera of pregnant women. These fractions coincided with each other. 2. Some of the blood sera of pregnant women of the 2nd-10th pregnant month gave precipitin reactions against to the fetal blood sera, especially the reactions were more and stronger in those of the 2nd-4th pregnant month, and that it was severer in case of pregnant toxemia, and moreover was the most manifest in case of the mother and child relations. 3. These facts were confirmed also from the side of passive anaphylactic phenomen.