Four antigenically related transmissible agents were recovered from canine fecal specimens. The agents produced cytopathic effects in a continuous dog cell line developed in this laboratory. Increased antibody titers were demonstrated in three of the four dogs which provided the isolates. The virus did not produce cytopathic effects in primary canine kidney or thymus cell cultures, or in cell cultures of human, simian, porcine, bovine, feline, and murine origin. The agent is resistant to ether, chloroform, and heat treatment, and the growth of the virus is inhibited by 5-iodo-2-deoxyuridine. After acridine orange staining, green fluorescent intranuclear inclusions are seen in infected cell cultures. By electron microscopy, the virions measure approximately 20 to 21 nm in overall diameter and are present in the nuclei of infected cells. These properties are consistent with membership in the parvovirus or picodnavirus group. The agent hemagglutinates rhesus red blood cells at 5 C and by hemagglutination-inhibition tests could be readily distinguished from H-1, rat virus, and the minute virus of mice. Canine gamma globulin contains high titers of neutralizing antibody and neutralizing antibody was found in a high percentage of military dogs and in beagles of a breeding colony.