The hemagglutinating properties of 10 viruses of the phlebotomus fever group were studied using antigens derived from infected newborn mouse brain tissue; the antigens were prepared by sucrose-acetone extraction and sonication. The hemagglutination reaction is practically not temperature-dependent. The pH range of the hemagglutination reaction for Bujaru, Iran-47, Karimabad and Salehabad viruses is 6.0 to 6.4, for Anhanga, Chagres, Naples and Sicilian sandfly fever viruses it is 6.0 to 6.8. The optimal pH for all viruses is 6.0 but for Anhanga virus it is 6.2. Treatment of erythrocytes with trypsin enhances their susceptibility in the hemagglutination reaction and extends the pH range of the reaction. The use of trypsinized erythrocytes permits to demonstrate the hemagglutination reaction with Punta-Toro virus which does not react with native erythrocytes. Erythrocytes of one-day-old chickens and rooster trypsin-treated erythrocytes might be used for hemagglutination reaction equally as well as goose erythrocytes; group 0 human erythrocytes and hamster erythrocytes are less susceptible. Chicken erythrocytes have some advantages in the hemagglutination inhibition test.