Large, negatively charged, multilamellar liposomes were examined for their ability to improve the therapeutic activity of the broad-spectrum antiviral agent ribavirin (RIB) and the synthetic immunostimulant muramyl tripeptide (MTP-PE) in the treatment of viral pneumonitis. Liposome-encapsulated MTP-PE (L-MTP-PE) was superior to free MTP-PE in activating alveolar macrophages and in protecting mice against intranasal challenge with 10 LD50 (50% lethal dose) of herpes simplex virus type 1 (HSV-1). Mice treated with liposome-encapsulated or free MTP-PE had no detectable viremia and had lower pulmonary titers of virus than controls. Liposome-encapsulated RIB (L-RIB; 3 mg per mouse), administered several hours after infection, was more effective than was free RIB (10 mg per mouse) in protecting mice against intranasal challenge with 10 LD50 of influenza virus, but neither L-RIB nor free RIB protected mice against HSV-1 infection. In contrast, combination therapy with both L-RIB and L-MTP-PE was more effective than either agent used alone.