The extremely tiny colony of B. influenzæ on ordinary blood agar makes it particularly difficult to detect, and one is apt to get the wrong impression of its numbers from the macroscopic appearance of the plate. In attempts at isolation there must be a liberal use of media in picking colonies, as many suspicious ones will turn out to be immature growths of B. xerosis, M. pharyngis (or M. catarrhalis), streptococci, or more rarely pneumococci and other organisms. Replating from such picks is frequently necessary, and further plates, from the original culture on heated blood agar, must often be made before the B. influenzæ can be isolated. The care required in all stages of the isolation of this organism, the unstinted use of media for plating and for picks, the number of stained smears to be studied, and the further transfers necessary to verify results, all these limit the amount of material which can be studied with any degree of accuracy. If further the streptococci, the pneumococci, the Gram negative cocci, the capsulated Gram negative bacilli and many others are to receive any attention, it can readily be appreciated that a few cases carefully studied are of far more value than a large number hurriedly examined in an uncertain routine. The pathological study of the same cases on which I have done the bacteriology will be found in Dr. Klotz’s paper in these communications, and I will merely refer to some of the bacterial findings in the sections of the lungs and bronchi. The more inclusive methods which have been used in attempts to determine the etiological factor in influenza we have been unable to attempt, but I will refer later in this paper to the findings of the investigations of others. Immunological studies have been limited to a few investigations on the presence of agglutinins, complement binding substance, skin reactions and the amount of complement present in the sera of certain patients. The epidemiological and clinical studies are reported by Drs. Johnston and Lichty in this series of reports. _Material Studied_ The material used in the study I am reporting included swabs from the large bronchi and fluid from the lungs and pleural cavities of 32 autopsies, as well as blood cultures from 22 patients and swabs from the nasopharynx of 31 individuals. Fifteen sera were tested for fixation of complement with an antigen made from several strains of B. influenzæ. Fourteen other sera were tested for agglutinins. Complement content was determined in the sera of 25 patients. Skin tests after the Von Pirquet method were done on 14 convalescents, and carefully stained nasopharyngeal smears without cultures were studied from 48 patients.