An H₅ strain of infectious canine hepatitis virus was isolated from an infected stray dog. This virus was isolated in dog kidney cell culture and subjected to passage through swine kidney cell cultures, so that it might be attenuated in virulence. As a result, the virus which had undergone 22 passages through swine kidney cell cultures was no more pathogenic for dogs. Further more, dogs inoculated with this virus exhibited an increase in neutralizing antibody titer to 125 or more three weeks after inoculation. They withstood challenge made by a strongly virulent virus strain after the establishment of immunity, without manifesting any clinical signs. An antibody survey was conducted on stray dogs in and around Kumamoto, by using this virus. It resulted in giving a positive rate of 37.3 per cent.