Propionibacterium acnes was isolated in pure culture from brain abscesses that occurred in two patients after intracranial surgery. Although such infections are usually associated with cerebrospinal fluid shunts and other foreign bodies, these cases clearly demonstrate the pathogenicity of this anaerobic diphtheroid in the absence of such. Progression of P. acnes infection in the central nervous system can be insidious. To treat such infections adequately, therapy cannot rely upon some standard antimicrobial agents. Metronidazole, which is useful against most anaerobic organisms, is not effective against P. acnes.