Electron microscopy of purified preparations of equine arteritis virus (EAV) revealed enveloped, spherical particles with an average diameter of 55 mμ. The envelope was found to carry tiny projections on the surface, 3 to 5 mμ in length. No detailed structure of an internal component could be seen. In sections of EAV infected BHK cells 24 hours after inoculation, viral particles were shown to bud from the cytoplasmic matrix into cisternae of the endoplasmic reticulum, the process starting in the vicinity of the Golgi apparatus. The mature particles had an average diameter of 50 mμ, with an inner core measuring 25mμ.