Killed bacterial cells of Propionibacterium granulosum KP-45 were found to be potent modifiers of non-specific resistance of mice against infection with either Listeria monocytogenes or Salmonella typhimurium. Injection of P. granulosum 7 days prior to infection resulted in marked splenomegaly. Disseminated acute inflammatory foci were found histologically in the liver. These mice were apparently less susceptible to infection, since definitely lower bacterial counts were determined. Simultaneous application of both P. granulosum KP-45 and L. monocytogenes resulted, however, in increased susceptibility of mice to infection, since definitely higher bacterial counts were found and the inflammatory reaction to infection was markedly enhanced.