Besides promoting nonspecific resistance to experimental infection, muramyl peptide immunomodulators also potentiate the effects of antimicrobial agents. These substances have been evaluated in combination with appropriate anti-infective agents in various models of bacterial, fungal, parasitic, and viral infection. For the combined regimens to be effective, the immunomodulator apparently must be administered prophylactically; anti-infective efficacy is then increased in situations where the muramyl peptide is capable of promoting at least some resistance to the infection independent of the antimicrobial agent. Adjunctive administration of the muramyl peptide may reduce the required dosages of potentially toxic antimicrobial agents and hence may lower the risk of toxic side effects. Because of the way in which they stimulate the inflammatory/immune systems, muramyl peptide immunomodulators may improve the outcome of therapy for infections in immunocompromised hosts. Initial clinical applications could involve the use of these immunomodulators in combination with antimicrobial agents as prophylaxis in high-risk individuals without overt infections.