An isolated mutant Japanese-encephalitis virus (JEV), preferably IC47 (ATCC VR2412), is claimed, which is differentiated from the corresponding wild-type JEV by replacement of an acidic amino acid (aspartic acid or especially glutamic acid) with a basic amino acid (arginine or especially lysine) at position-138 in its envelope protein. Also claimed are: (a) an isolated DNA molecule coding for the genome of the mutant JEV, where the mutant JEV is differentiated from the corresponding wild-type JEV by a mutation consisting of replacement of one or two nucleotides in the gene coding for the envelope protein; (b) a method of producing the mutant JEV in cultured animal cells; (c) a method of producing neutralizing antibody to JEV by administering the mutant JEV into an animal; and (d) mutant flaviviruses with an acidic amino acid replaced by a basic amino acid in their envelope protein. The DNA molecule preferably consists of plasmid pM443 (ATCC 75491) cDNA inserted into it. The mutant virus is suitable for stimulating immunity to JEV in animals.