The leukocyte alkaline phosphatase(LPA) activity in vaious stage of Jap. Encephalitis, which was admitted to Busan National University hospital from July to September in 1978, was investigated histochemically with the Tomonaga's naphthol AX-MX phosphate fast blue RR method. The author measured and compared with circulating leukocyte. Neutrophil and LAP activities. Patients were investigated the difference of sex and age, and compared to LAP activity and the prognosis of Jap. Encephalitis. The results were summerized as follows. 1. The Scores of LAP activity in acute of Jap. Encephalitis(3.22) were significantly higher than that of mid stage(2.48), recovery stage(2.37) and the control group(2.24). 2. The LAP activity was not influenced by sex and age. 3. The increas of DAP activity was found in the patients with leukocytosis charaterized by neutrophilia and it was relatively parallel with the increase of circulating neutrophil count. 4. In comparison to LAP activity and the prognosis in Jap. Encephalitis, ingeneral, LAP activity was relatively increases as the prognosis was become poor, but it was insignificant, statistically.