Adenovirus particles have a highly ordered structure and are composed of protein and DNA. Human adenoviruses contain about 87% protein and 13% DNA (Green and Pina, 1963), while the larger avian chick embryo lethal orphan (CELO) virus consists of 83% protein and 17% DNA (Laver et al., 1971). In virions, the viral DNA is tightly associated with several virus-coded proteins. Disruption of virions with acetone, urea, or pyridine, or repeated freezing and thawing, releases the viral cores, which, in addition to the viral DNA, still contain about 18–20% of the total protein of the virions (Laver et al., 1967, 1968; Maizel et al., 1968; Prage et al., 1968, 1970). The proteins found in viral cores are mainly two basic polypeptides. The major core protein is identical to polypeptide VII [molecular weight 18,000 (18K)], of which about 1000 copies are present in each viral particle. The minor core protein is polypeptide V (molecular weight 45.5K), of which each virion contains about 200 copies (Laver et al., 1968; Prage et al., 1968, 1970; Prage and Pettersson, 1971; Russell et al., 1971; Everitt et al., 1973; Laver, 1970). However, when cores are prepared by extraction of virions with sarkosyl, only polypeptide VII is found associated with the DNA (Brown et al., 1975). The different protein compositions of pyridine and sarkosyl cores suggest that polypeptide VII is more intimately associated with the viral genome than is polypeptide V.