The hemagglutination activities of Hantaan virus and Seoul virus were demonstrated. The hemagglutinins were prepared by sucrose-acetone extraction method from suckling mouse brains infectecd with Hantaan and Seoul viruses. Hemagglutination of goose erythrocytes by these viral hemagglutinins was pH dependent in phosphate buffer system. Hantaan and Seoul viruses were distinguished by pH range of hemagglutination. 76/118 and 79/90 strains of Hantaan virus showed hemagglutination at the range of pH 5.75-6.4 and the optimal pH was 5.75 with the titer of 1:512 in 76/118 and 1:256 in 79/90. In contrast, KSNUSD 84/34 strain of Seoul virus revealed hemagglutination at the range of pH 6.2-6.4 and the optimal pH was 6.4 with the titer of 1 : 64.