An American (Bucyrus) and a Swiss (Bibuna) strain of Equine Arteritis Virus both had a particle size lying in between 50 and 100 mμ, were resistant to treatment with trypsin, were quite sensitive towards heat and low pH, and were completely inactivated by molar MgCl2. When added to properties established earlier for EAV by the author (content of RNA, sensitivity towards lipid solvents) these features do not easily classify EAV whithin one of the presently defined groups of viruses. Emphasis is put on similarities between EAV and the viruses of hog cholera, bovine virus diarrhea, and eventually human hepatitis. All four agents might be members of a group of hemoviruses.