The injection of virulent cultures of Bacillus influenzæ into the subdural space of several species of lower monkeys is followed by the development of an acute inflammation of the meninges, corresponding in clinical, bacteriological, and pathological effects with influenzal cerebrospinal meningitis in human beings. Experimental influenzal meningitis in the monkey is a lethal disease which terminates fatally in from thirty-six hours to four days after the inoculation. The injected influenza bacilli produce their effects through multiplication in the course of which they penetrate from the subdural space into the general blood current, from which they may be recovered during life and at autopsy, as is also true of the spontaneous form of influenzal meningitis in man. By repeated injection, over a period of many months, of living virulent cultures of Bacillus influenzæ into the goat, an immune serum possessing moderate agglutinating and high opsonic power may be produced, which is capable, when injected into the subdural space, of arresting the progress of an experimental influenzal meningitis, and of bringing about recovery in monkeys thus affected. As a result of the serum injections, the influenza bacilli in the meninges are more freely englobed by phagocytes, their number is reduced, their capacity of growth diminished, and the eruption into the blood arrested. Along with these effects go, hand in hand, cessation of the local inflammatory process and progressive amelioration of the symptoms of illness, to be followed usually by rapid restoration of health. In view of the highly fatal character of influenzal meningitis in human beings, the employment of an immune serum by subdural injection is recommended. Undoubtedly it will be necessary to apply the serum early and by repeated injections, by means of lumbar puncture, to secure beneficial results. The early application will, in turn, be dependent upon prompt bacteriological diagnosis, which can be made, as a rule, by the immediate microscopical examination of the cerebrospinal fluid without the employment of cultural methods. When possible, the microscopical diagnosis should be confirmed by cultural tests.