Because of the wide variations in opinion concerning the relationship of various bacteria to influenza that have arisen during the progress of the recent pandemic, a brief review of the salient features of the earlier literature seems advisable. In 1892 Pfeiffer[8] found a small, Gram-negative, hemophilic bacillus in all cases of influenza, often in almost pure culture, both during life and at autopsy. He stated that the organism was found only in cases of influenza or in those convalescent from the disease. Similar bacilli occasionally found in other conditions he classified as pseudoinfluenza bacilli. He furthermore showed that freshly isolated cultures were pathogenic for monkeys, producing a disease not unlike influenza, though lacking in what he considered the characteristic lung lesions. He therefore felt justified in claiming that this bacillus, which he designated B. influenzæ, was the cause of epidemic influenza. Pfeiffer’s work, though hailed by many as unassailable, has failed to stand the test of time in two respects. It has been definitely shown, by Wollstein[9] in particular, that there is no justification for recognizing a group of pseudoinfluenza bacilli, organisms so classified by Pfeiffer being indistinguishable from B. influenzæ. Furthermore, numerous investigations have demonstrated that B. influenzæ may frequently be found in a variety of diseases affecting the respiratory tract and in a small proportion of normal individuals. Kretz[10] found it 47 times in 950 examinations, usually associated with disease of the respiratory tract. Süsswein,[11] Liebscher,[12] Jehle,[13] Wollstein,[9] Davis[14] and many others have demonstrated its presence in cases of measles. Lord[15] isolated B. influenzæ in 30 per cent of 186 sputums from patients with acute and chronic infection of the respiratory tract. Boggs[16] found it in frequent association with chronic bronchiectasis. Wollstein[9][17] showed that it was often present in the respiratory diseases of infants, and was not an infrequent cause of meningitis. Rosenthal[18] found that one in six of normal individuals harbors influenza bacilli and therefore considered it purely a saprophyte, a position, of course, thoroughly untenable in the face of indisputable evidence that it may be highly pathogenic. The widely accepted statement that B. influenzæ is nonpathogenic for animals has apparently served in considerable degree to shake belief in its etiologic relationship to epidemic influenza. It would appear, however, that this opinion is not founded upon fact. Reference is again made to the work of Wollstein[19], who has shown that virulent strains of B. influenzæ, when freshly isolated from the human host, are highly pathogenic for rabbits and monkeys and that nearly all strains are more or less pathogenic for mice and guinea-pigs.