Cytopathic changes in LLC-MK2 cells infected with SV4 virus, observed with the electron microscope and using acridine orange staining and fluorescent microscopy, have been shown to be similar t o that caused by picornaviruses and members of the Columbia-SIC virus group. The virus was found t o be stabilized against heat in the presence of molar magnesium chloride, and to be stable a t pH 3.5. The virus was non-pathogenic for suckling mice, failed to agglutinate sheep and human "0" red blood cells, but agglutinated rhesus monlcey erythrocytes a t 4 "C. On the basis of these properties and those already known, it was suggested that SV4 virus be placed in the group Enteroviruses of lower animals.