Semliki Forest virus preparations of different degrees of purification were sprayed into atmospheres of different relative humidity. Inactivation of airborne virus was greatest at high relative humidities and it decreased gradually as the relative humidity was decreased. Removal of salts from the sprayed suspension resulted in improved survival over the whole range of relative humidity tested. Extraneous protein was essential for survival at high relative humidities. Polyhydroxy-compounds protected airborne virus very well in conditions of low relative humidity.