An impression has been gained by clinicians after many years of observations that a number of infectious diseases occur with notable rarity during pregnancy (1). This physiological protection which nature has temporarily provided for the expectant mother extends also into the first few weeks of life of the newborn, only to be followed shortly later by a period of maximum susceptibility to the common infections of childhood. The mechanism of this spontaneous and transitory phase of enhanced resistance is not clear, neither in the mother nor in the baby. The constitutional conditions, however, which are associated with the stated epochs of life, must evidently be of fundamental character since several examples have become known of an increase in the titer of certain normal protective principles in the serum during the period of gestation. We refer here only to the increase in anthrakocidal substances (2) and in dysentery agglutinins (3) in the serum of pregnant women.