Spherical dark inclusions were observed in both the cytoplasm and nuclei of infectious canine laryngotracheitis (ICL) adenovirus infected MDCK (Madin–Darby Canine Kidney) cells. The distribution of these inclusions in the cells appeared to indicate that they were formed in both the cytoplasm and in the nucleus at about the same time and there did not appear to be movement of these inclusions between the cytoplasm and the nucleus during the early stages of infection.Morphological appearance, 3H-leucine autoradiography, and immunoferritin labelling showed that the cytoplasmic inclusions were similar in nature to the dark nuclear inclusions, and contained the adenovirus hexon antigen, but not the penton base and fiber antigens.