It is, of course, perfectly possible on the basis of the observations presented still to regard B. influenzæ as a secondary invader which makes its appearance in all cases of influenza simultaneously with the onset of clinical symptoms. Final proof of its causal relationship to the disease must depend upon the production of influenza by experimental inoculation. Results hitherto obtained in attempts to produce the disease experimentally have been contradictory. Pfeiffer[8] claimed to have produced a disease in monkeys in some respects resembling influenza by the intratracheal injection of freshly isolated cultures of B. influenzæ. Wollstein,[19] in studies upon the pathogenicity of various strains, has shown that B. influenzæ is generally pathogenic for mice and guinea-pigs without respect to source or virulence for man. Pathogenicity for rabbits and monkeys, on the other hand, was possessed only by strains that were highly virulent for man. She furthermore pointed out that for successful animal experimentation, it is imperative that inoculations be carried out immediately after the isolation of the bacilli because they rapidly lose virulence by subculture on artificial media. It is felt that failure to appreciate these facts has been responsible for the often repeated statement that B. influenzæ is not pathogenic for animals. In a series of animal experiments carried out by this commission recorded in an appendix to this report, sixteen-hour cultures of B. influenzæ freshly isolated from early cases of influenza were demonstrated to be pathogenic for monkeys, both by inoculation of the nasal and pharyngeal mucosa and by intratracheal injection. Monkeys so inoculated developed coryza, epistaxis, tracheitis, bronchitis, and extreme prostration. Experiments with forty-eight-hour cultures of strains preserved by subculture during from ten to fifteen days failed to demonstrate pathogenicity for monkeys. Proof that these monkeys had influenza can depend only upon the demonstration that they suffered with a disease having the clinical character and pathologic lesions of influenza.