Levamisole, a synthetically produced anthelminthic agent, was administered to suckling rats that were challenged with bacterial and viral pathogens. There was no in vitro antibacterial or antiviral effect of the drug. However, enhanced survival was demonstrated in those animals treated with levamisole and challenged with Staphylococcus aureus or herpes simplex virus. The mechanism for protection against the bacteria appears to be enhancement of the primary inflammatory response. The mechanism for protection against herpes simplex virus is not yet understood.