1. Quantitative studies of the skin-sensitizing antibodies, blocking antibodies, and hemagglutinins in sera of allergic human beings have been made. 2. A comparison of 12 maternal and the 12 corresponding cord sera by the method of passive transfer showed the human placenta to be impermeable to skin-sensitizing antibodies. 3. Direct skin tests and passive transfer studies of 6 infants at the ages of 3 to 6 months showed negative reactions to the antigens to which their mothers were sensitive. 4. The blocking antibody present in the sera of hay fever patients after treatment with pollen extract injections was also demonstrated in the cord sera. The apparent placental transmission of this antibody gave further evidence that it was distinct from the skin-sensitizing antibody. Infant sera obtained at the ages of 3 to 6 months showed no evidence of this immune antibody. 5. The cord sera from 4 of these cases were shown to contain the same isoagglutinins as the maternal sera, showing that the placentas were permeable to these antibodies. The mothers and their offspring reacted alike to Schick testing. 6. Typhoid agglutinins were demonstrated in maternal and cord sera of an adult who previously received injections of triple typhoid vaccine, whereas the serum of the corresponding infant at the age of 3 months failed to show agglutinins.