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, E-138, 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), E-176 (He→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. 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 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 (ic) or intraperitonial (ip) inoculation of 10-12 g mice. , SA14-14-2 vaccine virus, SA14-9-7 and SA14-5-3, did not exhibit lethal infections by ic or ip inoculation of 10-12 g mice and revert to the virulence. The other virus strain, SA14- 12-1-7, no neuroinvasiveness by ip inoculation but residual neurovirulence by ic inoculation and reverted to high virulence after one brain passage. Comparison of the E protein gene sequences of the five virus strains indicates that there are differences of twelve nucleoti des 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, E-138, E-279, E-315) 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), E-176 (He → Val), and E-439 (Lys → Arg) 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.