Resistance to a representative group of experimental viral infections in mice was significantly enhanced by nonspecific modulation of host defense mechanisms. Corynebacterium acnes, Corynebacterium parvum, and bacille Calmette-Guerin were effective in enhancing host resistance. Animals treated seven to 10 days before inoculation of virus were protected against a lethal infection with Herpesvirus hominis type 2, encephalomyocarditis virus, murine cytomegalovirus, or Semliki Forest virus. The protection of experimental animals against encephalomyocarditis virus infection initiated by either the intraperitoneal or the respiratory route indicated that C. aenes exerted a systemic, rather than local, effect. A maturation process was required for host defense mechanisms stimulated by C. acnes, as indicated by the failure to enhance resistance in suckling animals. Involvement of cells of the lymphoreticular system was demonstrated by transfer of enhanced resistance against H. hominis type 2 to recipient animals with peritoneal exudate cells harvested from mice pretreated with C. acnes. Finally, these same cells inhibited the progression of herpetic infection in tissue culture. The data suggest that immunomodulation, possibly through activation of macrophages, may offer a method for enhancement of host resistance to viral infections.