Attenuation of Japanese encephalitis virus by brain passage of guinea pigs.

JE virus was difficult to keep alive in guinea pig brain beyond 3 days and the virus was lost after 3 days. Ten cross passages from guinea pig to mice attenuated JE virus (GP10) and the result indicated that 3 passages could induce illness in guinea pig (GP4). GP4 and GP10 did not induce viremia to mice. 0.1 ml of 1:100,000 and 1:10,000 mouse suspension of GP4 inoculated into the author subcutaneously did not cause any uncomfortable symptom.