This article is only available in the PDF format. Download the PDF to view the article, as well as its associated figures and tables. In this study, begun in 1928, only selected young nonimmune children are included. Their past medical histories were familiar to me from their infancy. Their ages ranged from 9 months to several years, with an average age of less than 3 years. For obvious reasons, those families were given preference in which there were also older or younger nonimmune children. During these four years I have given more than 1,600 injections to about 300 young children without causing any severe local or systemic reactions. A fresh supply of the vaccine was prepared every few months by Miss Leonora Hambrecht, A.B., my collaborator in other researches in whooping cough. Each time from five to seven strongly hemolytic strains of Hemophilus pertussis (B. pertussis of Bordet and Gengou) were used. These smooth type organisms were isolated from early cases by the cough plate method, shortly before each lot of vaccine was prepared.