A low-molecular-weight chemical inducer of interferon, 10-carboxymethyl-9-acridanone (CMA), effectively prevented death caused by Japanese encephalitis virus (JEV) injected peripherally into weanling mice and baby hamsters. Marked reductions in mortality were seen in mice when a single dose of CMA was administered intraperitoneally, subcutaneously, or intramuscularly to animals challenged intraperitoneally with JEV. The degree of protection was dependent on dose and time of administration of CMA in relation to virus challenge: all hamsters given CMA on the same day as JEV survived, with lesser although still significant protection when CMA was given one or two days after JEV. Viremia, an important characteristic of the pathogenesis of natural JEV infection, was reduced nearly 10,000-fold in hamsters treated with CMA. Thus, in the experimental animal models developed for these studies, CMA provided marked therapeutic and prophylactic effect against JEV.