Immunization against whooping cough has been a routine procedure at the Evanston Health Department clinic and at St. Vincent's Infant and Maternity Hospital (Chicago) since 1933. At first the primary object was to collect data on the relative merits of various concentrations of Hemophilus pertussis vaccine and on the optimum time interval between the three doses. During the past six years the purpose has been to obtain data on the potency of alum precipitated vaccine (plain), in combination with diphtheria toxoid, or with diphtheria and tetanus toxoids. Within a few years after these prevention clinics were established, the incidence of whooping cough decreased greatly among the children injected at Evanston, and since 1935 has ceased entirely among the children injected with H. pertussis vaccine at St. Vincent's. It became evident that a reliable test for vaccine conferred immunity was necessary. If an injected vaccine was potent there should be a