Adenovirus type 5 'cores' prepared by heating in the presence of deoxycholate and partially purified on a glycerol density gradient could be visualized as roughly isometrical particles with a condensed centre from which twisted filaments or loops of DNA emanated. This compact structure was readily dispersed by spreading on distilled water or by treatment with EDTA, Nonidet, DNase or trypsin. Spreading with Nonidet was particularly effective in unfolding the cores and revealing long filaments about 100 A thick presumably of the virus nucleoprotein. Subunits (about 30 to 60 A in diam.) could be seen free in the DNase-treated cores, suggesting a particulate nature of one or both of the core proteins.