Although many antibiotics against bacteria were so far isolated as products of streptomyces, substances effective against small viruses, such as small pox, influenza, poliomyelitis and Japanese encephalitis viruses in vivo, were only infrequently reported. No agent, whether chemicals or antibiotics, could ever protect the animals from infections of small viruses.

Among substances described to possess an antiviral activity,^{1),2),5),8),11),16)–43)} only achromoviromycin,¹⁾ an antibiotic, and PANS,²⁾ a chemical, were described to have an activity against Japanese B. encephalitis (JBE). The latter found a clinical application, though its effect is not decisive.

In our laboratory, soil isolates of streptomyces were tested for their activity against JBE virus in mice, and culture filtrate of streptomyces was proved to protect the mice against the infection with JBE virus. From the filtrate, a new effective substance was isolated and was given the name of cephalomycin.³⁾ This paper deals with the experiment in mice infected with JBE virus, and the experiments for protection against reinfection with JBE in the surviving mice.