The lipase activities in 55 materials of liquor cerebrospinalis taken from 27 Japanese B encephalitis patients, admitted to the attached hospital of this school were investigated over a period of 2 years, 1952-1953. 1. The lipase activity of the liquor cerebrospinalis of Japanese B encephalitis patient was ascertained, in most cases, to be low compared with those of the patients of other diseases like those that affect the menix, the brain, or the spinal cord. This is not quite sufficient to differentiate the diagnosis of the Japanese B encephalitis disease from the others cited above. 2. No such appreciable amount of changes in the lipase activity as might he expected in connexion with the length or the gravity of the diseases were fuund, which indicates that the lipase activity cannot help to determine the prognosis.