According to Wright1 diphtheria bacilli are insensible to the opsonic action of blood fluids. This conclusion is based upon experiments showing that there is as much (or, in one case, more) phagocytosis in the presence of normal human serum heated at 600 C. as in unheated. In order to study this question four strains of typical diphtheria bacilli furnished me by Dr. Hamilton were tested. With all of these strains there was no opsonin demonstrable in normal human serum after heating the serum 30 minutes at 500 C. This was found to be true also of serum from diphtheria patients when tested with strain "N," the organism employed in the subsequent experiments. Re que2 also found that when washed leucocytes are mixed with normal human or dog serum heated to 58-600 for 15-30 minutes, there 1s practically no phagocytosis with diphtheria bacilli. It is possible that the strain which Wright used was subject to the spontaneous phagocytosis which has been observed to occur with occasional strains of diphtheria bacilli. As the course of the specific opsonic index in certain acute infections has been found to possess characteristic features that harmonize well with the general clinical picture, rising above normal as the symptoms decline, it was thought it would be of interest to determine if this were also true in regard to diphtheria. Both the diphtheriaand the streptococco-opsonic indices were estimated on account of the close association of streptococci with tonsilar infections. The Wright method of estimating the opsonic index has been employed for the most part. The suspensions are made from 24,