Studies in a number of laboratories have indicated that the internal protein components of adenovirus are basic in nature and possess some properties which are similar to the histones and protamines. Thus, they are acid-soluble, rich in arginine and have alanine as their major N-terminal amino acid (Russell, Laver & Sanderson, 1968; Prage, Pettersson & Philipson, 1968; Maizel, White & Scharff, 1968; Boulanger et al. 1970; Laver, 1970; Russell, McIntosh & Skehel, 1971). The possible function of these components in infection is of great interest; furthermore, they provide a relatively simple system in which to study the interaction of basic proteins and nucleic acids. As one approach to this problem, the changes in the metabolism of cellular histones as a result of infection by adenovirus have been examined by analysing the acid-soluble components of infected and control cells in polyacrylamide gel electrophoresis.