We reported in a previous paperla that protective antibodies were found in a large percentage of patients with pertussis who whooped three weeks or longer and that these antibodies were found only occasionally earlier in the disease. Agglutinins were found at that time in less than one half of the patients tested, and the agglutinin titers of these patients did not correspond either to the duration of the illness or to the protective antibody titers of their serums. In continuation of these studies, it seemed important to determine: (1) whether pertussis-protective, agglutinating and complement-fixing antibodies are present in persons with no history of pertussis and no symptoms of illness; (2) whether the aforementioned antibodies are found late in the course of pertussis as well as during the early stage; (3) whether these antibodies are stimulated in persons who are intimately exposed to persons with active pertussis but do not