It is now twenty-four years since Bordet and Gengou isolated the causative organism of pertussis, and fourteen years since Chievitz and Meyer1introduced their simple method of early diagnosis. Epidemiology has been slow to evaluate these fundamental discoveries. Despite the fact that the cause is known, pertussis remains nearly as prevalent as measles, the cause of which is not known. No serious contagious disease of childhood is diagnosed with more uncertainty and tardiness or is more frequently left undiagnosed than is pertussis. As a rule, diagnosis is delayed until the whoop is well established, and quarantine is seldom observed until after the period of greatest contagion. In a disease so fatal to the young, causing a mortality approaching that of diphtheria and about twice that of scarlet fever, quarantine should be established as early as possible. The cough plate is the best means of early diagnosis. It has been