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Five strains of Bacillus pfeifferi were isolated from a case of meningitis. These strains came from the spinal fluid, blood, nose, throat, and nasopharynx. Immunologic reactions show no definite relations between these strains, although those from the nose, throat, and nasopharynx might be presumed to be related to one another. It is also presumable that the spinal fluid strain was derived from the upper air passages, and that the blood was invaded from the meninges. In spite of immunologic differences, it is believed that the five strains were genetically related. The variations in these five strains from a single case are as great as those which have been found by others for strains from different cases, or individuals. As determined by immunologic reactions, the number of so called strains of Bacillus pfeifferi is apparently limited only by the number of cultures which have been or might be isolated. It is inconceivable that under the designation Bacillus pfeifferi is included a heterogeneous mixture of innumerable distinct races of bacteria; there must be some biological relation which cannot be established by agglutination, complement fixation, or opsonin reactions. The variations which have been noted, may be an indication of a degree of instability which may have some bearing upon questions relating to the epidemiology of Bacillus pfeifferi infections and to the serum therapy of influenzal meningitis. The strain variations which have been shown to exist by immunologic methods do not support the theory of the etiological relationn of Bacillus pfeifferi to epidemic influenza, unless it can be shown that such variations are due to instability of the organism. The fact that the incidence of meningeal involvement was only slightly increased during the last pandemic is also evidence against the causal relation of the organism to the pandemic disease.