It has long been a matter of common observation that, although newborn babies have a high degree of resistance to such childhood diseases as measles, scarlet fever and diphtheria, they appear to be susceptible to whooping cough from the day of birth. Pertussis in the newborn is especially severe, and fatality rates are highest in babies under 6 months of age. According to Dauer,1 in the period 1935 to 1939, 40 per cent of the deaths from whooping cough in the Registration Area were of babies under 5 months of age. This high proportion of deaths is of particular significance in view of the low incidence of the disease in this age group. Resistance to scarlet fever, measles and diphtheria is generally explained on the basis of a passive immunity acquired by way of the placenta, this immunity, of course, being gradually lost during the first few months of