Thermal and pH stability of ephemeral fever virus (EFV) were studied. Infectivity of EFV was inactivated within 10 minutes at 56°C, by 18 hours at 37°C, and within 120 hours at 25°C. Infected mouse-brain virus had little titer loss after 30 days' storage at 4°C. Infected culture fluids stored at −35°C lost about 0.04 log units of virus per day. EFV was inactivated within 10 minutes at pH 2.5 and 12.0. Infectivity was lost in 60 minutes at pH 5.1 and in 90 minutes at pH 9.1. EFV was adapted to monkey kidney cell lines (Vero and MS); a plaque assay in Vero cells is described. The virus sized between 100 and 220 mμ by filtration. Growth studies of EFV in MS cells are described.