A series of thirty-five patients suffering from diphtheria, who had been previously fully inoculated against the disease and from whom toxigenic diphtheria bacilli were isolated, were investigated and compared with seventeen patients who had not been inoculated. There were smaller groups of those patients who had had incomplete courses of immunization and of those who had had previous attacks of the disease.

Two-thirds of the infections in the inoculated were due to Corynebacterium diphtheriae, intermedius type. Intermedius infections were proportionately more numerous in the inoculated than in the uninoculated, whereas gravis infections were less frequent.

Estimations of the antitoxin in the blood were made on admission. The inoculated patients had levels of antitoxin below, or in the neighbourhood of, that believed to confer protection.

In the inoculated group severity did not appear to be influenced by the level of antitoxin in the blood on admission.

Antitoxin in the blood was found to have increased after recovery from the disease in all except one patient; the increase was considerable in those who had been inoculated previously.

From 23·5% of all cases of clinical diphtheria investigated only non-toxigenic diphtheria bacilli (gravis, intermedius and mitis types) were isolated. These cases were considered separately. On admission the majority had in their blood considerable amounts of antitoxin, which did not increase after recovery. No other pathogenic organisms were regularly found. The possibility is discussed that under certain conditions non-toxigenic C. diphtheriae may be able to cause a diphtherialike disease.