The titer of diphtheria antitoxin in maternal and infant (cord) blood was compared in 56 premature infants. A significant correlation was found between birth weight and placental permeability to diphtheria antitoxin. Small premature infants (under 1200 gm.) have much less circulating antitoxin derived from the mother than do larger premature infants. In 10 premature infants with high cord titers of diphtheria antitoxin, this passive immunity was lost at about the same rate as by normal newborn infants.

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