Immune RNA (iRNA) preparation was extracted with phenol from the spleens, lymph nodes and livers of rabbits immunized with Japanese Encephalitis virus (JEV). When C57BL mice were injected intraperitoneally with this preparation, the serum antibodies were detected by complement fixation test, indirect immunofluorescence technique and neutralization test. A markedly higher of ser fjter um antibody was demonstrated by all these three methods in such iRNA treated mice as compared with non-immunized or nRNA treated mice. There is no significant difference between spleen-lymph nodes iRNA and liver iRNA in their antibody induction activities. The titer of serum antibody was greatly increased by a previous injection of a sub-threshold amount of JEV vaccine into mice which subsequently received an injection with iRNA. Adoptive transfer of spleen cells which had previously sensitized with iRNA into mice had the same effect on antibody formation as direct injection of iRNA.