The largest colonies of the bacillus I have obtained were those growing near the periphery of a colony of an air nocardia. I have also noted that emulsions of a staphylococcus killed by boiling for five minutes, when added to ordinary blood agar, had a marked stimulating effect, although no evidence of hemolysis was present. This effect was practically absent if the emulsion was boiled for 15 minutes, or after being killed was left at room temperature for several days. There was no evidence of these stimulating effects by any of these methods when heated blood agar was used, the colonies on this medium growing equally large by themselves. Comparative studies of the effect of different bacteria can be simply carried out as follows: Smear evenly the surface of an ordinary blood agar plate with an emulsion of B. influenzæ. Seed this plate at various points with minimal amounts of the various bacteria. After various periods of incubation the size of the B. influenzæ colonies about the other bacterial growths can be estimated, and impression preparations on cover glasses will give very interesting pictures. The growth of B. influenzæ in primary cultures from sputa and similar sources is to be explained by the probable presence of traces of blood or altered hemoglobin as well as the symbiotic relationship with other bacteria. Fichtner used fresh heated sputum (60 to 65° C.) in place of blood, and Richter a medium made with sterilized pus. Parker, in her study of a filterable poison produced by the B. influenzæ, found veal infusion broth with 10 per cent. defibrinated blood heated to 75° C. until the blood coagulated and settled on standing to be the best for the purpose. Jordan in his study of indol production by these bacteria used a meat infusion broth with 5 per cent. sheep’s blood added at 90° C. or over and filtered through cotton or paper. Wittingham and Sims noted that in using blood from influenza cases the bacteria frequently did not grow, more especially B. influenzæ; and Rivers found human blood poorer than cat or rabbit blood for growing this organism, as did Minaker and Irvine. It would seem clear from this review of some of the suggestive work on the methods of growing B. influenzæ that little attention should be given to the results of many workers, where ordinary media were used, particularly when the difficulties of isolation were not appreciated. _B. Influenzæ as a Pathogenic Bacterium_