Intraperitoneal administration of 10-carboxymethyl-9-acridanone sodium salt (CMA) protected at least 50% of mice tested from otherwise lethal infections with Semliki forest, coxsackie B1, Columbia SK, Western equine encephalitis, herpes simplex, and pseudorabies viruses. The protective effect against influenza A2/Asian/J305 and coxsackie A21 viruses was less but was statistically significant. When administered either subcutaneously or orally, CMA protected at least 50% of mice against Semliki forest and pseudorabies viruses; the effect against coxsackie B1 and herpes simplex viruses was less but was statistically significant. Initiation of treatment could be delayed from 2 to 24 h after infection of mice with coxsackie B1, herpes simplex, Semliki forest, and Western equine encephalitis viruses without loss of an antiviral effect. CMA did not inactivate Semliki forest or coxsackie B1 viruses on contact and was without effect against any of the viruses tested in tissue culture by the tube dilution assay. The humoral antibody response in mice to both influenza virus and sheep erythrocytes was unaffected by CMA. After administration of CMA, an interferon-like substance was induced in mice or mouse cell culture but not in rabbits or rabbit cell culture.