Using fluorescent antibody techniques (FA) and light microscopy (LM) and electron microscopy (EM), this paper describes the morphological features of the ileum in the DK1 mouse orally challenged with adenovirus K87. At the peak of infection, virus is easily identified by FA in the epithelium of the villi and crypts of the ileum. LM shows that fluorescent cells have large, bizarre, uniformly basophilic nuclei containing deoxyribonucleic acid, as indicated by histochemical tests. EM further identifies these nuclei as belonging to columnar, goblet, or Paneth cells, all epithelial cells facing the lumen with a microvillus border. The basophilic material in the nuclei consists of virus particles 75 nm in diameter arranged in crystalline arrays. When found in the cell cytoplasm, the virions do not form arrays but are scattered or form irregular aggregates, which may or may not be enclosed by single membranes. Infected columnar cells show mild cytopathic effects with no cell degeneration and necrosis, whereas the goblet and Paneth cells appear normal and maintain synthetic and secretory functions. All infected cells, however, share an abnormally accelerated extrusion rate, with columnar and goblet cells often being shed from the side rather than from the tip of the villi. The Paneth cells, which do not migrate out of the crypts, show a higher than normal rate of extrusion in the crypt lumen.