The morphology, protein composition and DNA organization of nucleoprotein core complexes isolated from type 5 adenovirions have been examined by electron microscopy and biochemical techniques. The morphology of such core structures is in some ways strikingly similar to that exhibited by cellular chromatin. 'Native' core preparations contain compact and less highly-folded forms: the latter appear as thick fibres, 150-300A in diameter. Upon exposure to 0.4M NaCl, adenovirus cores undergo a transition to a beaded string form, reminiscent of nucleosomes. Of the three arginine-rich proteins, polypeptides V, VII and mu present in 'native' cores, only polypeptide VII remains associated with viral DNA in the presence of 0.4M NaCl. We therefore conclude that the nucleosome-like beads are constructed solely of polypeptide VII. The results of micrococcal nuclease digestion experiments suggest that polypeptide VII is sufficient to protect some 100-300bp of adenoviral DNA.