If B. influenzæ is the causative agent in clinical influenza, there is certainly ample evidence that it is pathogenic to man. The symptoms of toxemia, which are so manifest in the pandemic disease as well as in the sporadic cases, would indicate that the etiological agent is markedly toxicogenic. Animal experiments by Pfeiffer, and a long list of investigators following him, would seem to show that the majority of cultures of B. influenzæ do not have any power of establishing themselves in the animal tissue. Killed cultures showed equally as high toxic effects as the living, and so it was generally concluded that many of the general effects in influenzal infections were of a toxic nature. There are many exceptions to the above-mentioned failures to produce infections in animals. Cantani obtained very constant positive results by subdural injections. He first clearly showed that killed cultures were markedly toxic and that virulence could be raised very definitely by animal passage. By injecting brain emulsion with a culture he obtained a subcutaneous abscess in a rabbit which after eight days still contained the living organism. Nastjukoff found that animals with a lowered resistance, or definitely ill from, for example, an artificial tuberculosis, became infected while others did not. Jacobson showed that B. influenzæ injected with streptococci caused a definite mixed infection, and that after six passages the influenza bacillus alone could produce a fatal infection. Saathoff (1907) confirmed Jacobson’s findings and found pneumococci equally effective. Davis (1915) also confirmed the principle established by Jacobson of the symbiotic relation of other bacteria to infection with B. influenzæ. He used a culture of a non-virulent staphylococcus pyogenes aureus, and was able to produce death invariably in guinea pigs after intraperitoneal injection. From the heart’s blood, as a rule, only the hemophilic bacillus was recovered. He also found animal passage increased the virulence, and further that M. catarrhalis and an avirulent streptococcus had the same effect as the staphylococcus. Slatineanu (1901) found that he could infect animals with B. influenzæ if the cultures were injected along with weak solutions of lactic acid, and that after animal passage by this method the bacillus became more virulent and would eventually kill by itself. It must not be forgotten in this connection that strains of B. influenzæ from meningitis cases are frequently definitely pathogenic for animals. The importance of considering these various factors in a discussion of infection by this organism is, of course, very evident. Ecker found his strains pathogenic for mice after subcutaneous injection, and the bacilli were readily obtained from the heart’s blood. Spooner and his co-workers from their results of more than a hundred intraperitoneal injections concluded that the organism is not pathogenic for mice.