In a previous communication1 we reported the results of a four year study of the routine inoculation with parental serum of every child admitted to the pediatric service. The findings indicated a definite diminution in the incidence of secondary infectious diseases, in contrast to the number which developed in the surgical pediatric ward, which served as a control.
The purpose of the present study was to determine, if possible, the exact nature of the protection which parental serum apparently conferred on its recipients. Because of the fact that susceptibility of a person to diphtheria can be determined within twenty-four hours by means of the Schick reaction, which is accepted as a criterion from the practical point of view, diphtheria was singled out for special study. Moreover, we felt that by studying the effect of human serum in a disease immunity to which can be measured on an accurate and