The studies on the effect of the Recombinant Leukocyte A Interferon (r IFN-alpha A) on 4 local strains of JE virus in Thailand were performed in vitro in our laboratories in Bangkok during August - October 1984. The procedures consisted of the plaque reduction assay and the Rhesus monkey kidney cell line, LLC-MK2 cells. These 4 strains namely Vip, KE-093, KE-094 and KE-095 were isolated from the JE patients in Thailand during 1983-1984. The results revealed that all of the JE virus strains tested were sensitive to the r IFN-alpha A with its minimal effective doses ranging from 30 I.U./ml to 1,500 I.U./ml. The studies on the effects of r IFN-alpha A on JE virus replicating in the cell culture for 0 hour, 1 hour and 6 hours indicated that if the virus had more hours to replicate in the cell culture, higher concentration of the IFN was needed in order to combat the replication of the virus in the cell culture. r IFN-alpha A at higher concentrations showed more efficacy in combating the replication of the JE virus in vitro.