Canine adenovirus type 1 (CAV-1), also known as infectious canine hepatitis (ICH) virus, and canine adenovirus type 2 (CAV-2) are both used for vaccination of dogs in the United States. Because of the close serologic relatedness between these two viruses, the canine adenoviruses (CAVs) are used interchangeably to vaccinate against each other with CAV-2 being the preferred agent since there are safety problems such as "blue eye" associated with the use of ICH vaccine. Although differences have been shown between CAV-1 and CAV-2 in morphology, pathogenicity in vivo and antigenicity, differentiation in vitro has not been successful and the immunologic distinctiveness of the CAVs has not been demonstrated such that each is still not officially classified as a separate CAV species (type);In the research described in this dissertation, in vitro type distinction and some strain differentiation of the CAVs important in vaccine production in the United States are shown by restriction endonuclease analysis of the genomes of the Utrecht (prototype), Lederle 255, Cornell-1-66, and Cornell-1-PK strains of CAV-1 and the Toronto A26/61 (Ditchfield) and Manhattan strains of CAV-2. Immunologic distinctiveness is demonstrated by a bank of type-specific monoclonal antibodies prepared against the Mirandola and Manhattan challenge strains of CAV-1 and CAV-2 respectively. Additionally, the structural polypeptides of these two viruses are described and characterized by sodium dodecyl sulfate (SDS) - polyacrylamide gel electrophoresis and radioimmunoprecipitation (RIP) with dog anti-CAV sera. RIPs with monoclonal antibodies are further utilized to define proteins of CAV that are important in type specificity, group specificity and neutralization.