The superhelical, closed circular form of polyoma deoxyribonucleic acid (DNA) (Co 1) is bound in a 25S DNA-protein complex to the viral histone-like proteins after alkaline disruption of the virion. Nicked viral DNA or linear DNA are largely free of protein. Most of the viral protein disruption is in the form of capsomeres, sedimenting principally at 10S and 7S. Despite the relatively constant ratio of 10S to 7S material in many preparations, (1:5.5 to 1:6.0, respectively), the two classes of capsomeres are indistinguishable by electron microscopy and contain only P(2), P(3), and P(4) in molar ratios of approximately 5:1:1 or 6:1:1, respectively. Material with sedimentation rates of approximately 1 to 3S is enriched for P(5) and contains small amounts of P(2), P(3), and P(4). During the in vitro reassembly of DNA-free, shell-like particles from disrupted virus, proteins P(1), P(2), P(3), P(4), and P(7) are reincorporated efficiently, whereas P(5) and P(6) are not. The presence in empty reassembled particles of histone-like protein, expecially P(7), implies that at least this one of the minor protein components of the virion may participate in protein-protein interactions with other components of the capsid.