This chapter describes the method of preparing and observing hepatocellular carcinoma (HCC) and surrounding normal liver cells infected with therapeutically p 53 -transfected adenovirus (Ad-p 53 ), so that morphology of the cells and viruses, and crucially their relationships to each other, are revealed. In standard practice, ultrastructural analysis of viruses carried in body fluids (e.g., stool or mucus) is sufficient for diagnosis, using the technique of phosphotungstate - dark field staining-of the aqueous extract. That method, however, is not suitable when one needs to examine precise subcellular location of viruses in situ, with tissue and cells intact, for complete pathological assessment; here, we describe our method (1 ) for transmission electron microscopy of the ultrastructure of virus-infected tumors. Tissue fixation, osmication, embedding, section cutting, and observation of Ad-p 53 infection will be included.