The experiments in man lead to very surprising results. Rosenau, Keegan, Goldberger and Lake, at Gallops Island, Boston, Mass., (1) inoculated volunteers with pure culture of B. Pfeiffer, with secretions of the upper air passages and with blood from typical cases of influenza. Sixteen men, of whom 13 were supposedly non-immune, had Pfeiffer bacilli installed into their nasal passages, and none of them developed the disease. Secretions filtered and unfiltered also gave negative results. Contact with well-developed early cases also failed. McCoy and Richey (1a) conducted similar experiments in San Francisco, with negative results. The men of the latter group had been vaccinated with a mixed streptococcic vaccine, which may have played some part. Had the experiments with the Pfeiffer bacillus been negative and the other experiments positive, they would have shown that the bacillus of Pfeiffer was not the cause of influenza; but since all attempts were negative, it merely brought out the fact that there had been a change, due probably to some immune factor, which seemed to have acted alike on the Pfeiffer bacillus and all other types of virus present, and to have made them all innocuous. These experiments still leave the cause of influenza in question. Those who are opposed to the Pfeiffer bacillus being the cause of influenza in its epidemic form base their position on the points that the common finding of the bacillus might be accounted for on the grounds of its being a secondary rather than a primary invader; that while it is not so common at ordinary times, it does occur with other organisms in whooping cough and sometimes in chronic diseases of the air passages, and that the rules of Koch have not been complied with in that the organism has not been found in every case of the disease; that where it has been grown in pure culture and inoculated into man and animals, it has either produced no disease, or the lesions which followed have not been typical of epidemic influenza. On the side of those who believe that the Pfeiffer bacillus is the chief cause, or, at any rate, that it is partly responsible for epidemic influenza, are the facts of its fairly constant presence in the purulent bronchial secretion of patients suffering from epidemic influenza; its relatively uncommon occurrence at other times; its known pathogenicity in occasional cases of meningitis, and in the inflammation of the bony sinuses of the head and face; the relative immunity of nearly all common laboratory animals and the fact that the attempts to transfer epidemic influenza from man to man failed not only when Pfeiffer bacilli were used, but also when direct contact and direct coughing by the patient into the face of the volunteer were tried. The argument that many cantonment laboratories failed to find the organisms loses weight when we find that the percentage of positives increased where the material examined was removed directly from the