Summary When purified preparations of adenovirus type 5 were heated the icosahedral capsid of the virus was ruptured at the twelve vertices, allowing the viral DNA to become accessible to the action of DNAse. The viral DNA could then be substantially separated from protein by digestion with trypsin or pronase. An analysis of the distribution of viral DNA by radioactivity and the various capsid complement-fixing antigens after density gradient centrifugation of heated virus suggested the presence of some other hitherto undescribed protein within the capsid.