The biochemical nature of adenovirus-induced inclusions was determined by specific enzyme digestion of thin sections combined with autoradiography using tritiated precursors.The early basophilic inclusions and the ring form inclusions were found to contain newly synthesized deoxyribonucleic acid (DNA), ribonucleic acid (RNA), and protein. The later appearing eosinophilic inclusions, both the electron light appearing ones and the electron dark appearing ones, consisted of protein.The total RNA synthesized by the infected cell was slightly increased; part of the increase was due to continuing synthesis of RNA by the nucleolus. The host DNA, after infection, was marginated along the nuclear membrane and was not subsequently incorporated into virus DNA.