The aim of this study was to explore the molecular basis for the attenuation of the Japanese encephalitis virus (JEV) vaccine strain SA14-14-2. The virulence of SA14 wild Japanese encephalitis virus (JEV) and its several attenuated viruses was tested by intracerebral (i. c. ) or intraperitonial (i. p. ) inoculation of 10-12 g mice. The stability of neuroattenuation was tested by one passage in suckling mouse brain. The E protein genes of those viruses were amplified by PCR, sequenced and compared. Three attenuated virus strains, SA14-14-2 vaccine virus, SA14-9-7 and SA14-5-3, did not exhibit lethal infections by i.c. or i.p. inoculation of 10-12 g mice and revert to the virulence. The other virus strain, SA14-12-1-7, showed no neuroinvasiveness by i.p. inoculation but residual neurovirulence by i.c. inoculation and reverted to high virulence after one brain passage. Comparison of the E protein gene sequences of the five virus strains indicated that there were differences of twelve nucleotides and eight amino acids between the parent strain SA14 and vaccine strain SA14-14-2, of which six amino acids (E-107, E-176, E-439, E138, E-279, E-315) exhibited changes common to those of SA14-9-7 and SA14-5-3, three substitutions common to SA14-12-1-7. Two amino acid substitutions at the sites E177 (T→A) and E264 (Q→H) are unique to the SA14-14-2 vaccine virus. The results suggest that the mutations of E-107 (Leu→Phe), E176 (Ile→Val), and E-439 (Lys→Arg) may contribute for the attenuation of neuroinvasiveness and partially for the attenuation of neurovirulence, the mutations of E-138, E-279, E-315 may not only critical to the neuroattenuation but also to its stability.