1) When rabbits are immunized by human fetal serum, high titer precipitin reaction on human serum can be obtained by injecting ten times. 2) When rabbits are immunized by human fetal serum with Globulin and Albumin precipitin productions can be obtained easier than that of human fetal serum. As for each antigenity, the remarkable difference can not be seen and the antigenity of fetal serum Albumin is by no means inferior to that of Globulin. 3) In case of fetal serum Albumin as antigen, precipitin quantity is lower than that of Globulin, and appearance of reaction is less. 4) Specific protein contained in fetal serum, exists in serum Globulin, not in Albumin.