In the year 1932 the use of alum-precipitated toxoid began to supplant the use of toxin-antitoxin and fluid toxoid for immunization against diphtheria. The simplicity of the single injection, its freedom from reactions in young children and the rapid development of immunity appealed to physicians and public health officials. It was assumed that the duration of the immunity was the same as that obtained by toxin-antitoxin, which Park and his co-workers had found to be ten years or more in 80 per cent of cases.1 Recently, however, several reports have appeared in the literature which suggest that the persistence of immunity (as determined by a negative Schick test) produced by alum-precipitated toxoid is not so long as was previously believed, although there are also other reports indicating the persistence of a high degree of immunity. A search of the literature has not revealed any studies covering over three years.