Abstract
The ratio of the titers of various antibodies, namely, hemagglutinins and syphilitic reagin, in the maternal blood to that of the corresponding antibody in the cord blood was found to be relatively constant, falling somewhere between 8 and 16. This figure may be considered the "index of permeability" of human placenta to antibodies, or the coefficient of distribution of antibodies between maternal and cord blood. The possible application of these findings to the study of the placental permeability to sensitizing antibodies (or reagins) is discussed.