A significant number of viruses produce intracellular changes, inclusion bodies, which do have specific and consistent features important to their identification. The inclusions must fulfill the designated criteria for the specific virus or a member of that group. The specific features of viral inclusions are consistent fixative-dependent artifacts: uniformity and familiarity of staining quality are important to the consistency of recognition and identification. Bouin's or Zenker's-acetic acid are the fixatives of choice, and a well-done hematoxylin and eosin stain offers the greatest uniformity of quality. Viral inclusions are not static objects in a cell and recognition of the range of development is often more important than the appearance of a single inclusion. Generally, most viruses within a group cause identical intracellular reactions and produce similar inclusion bodies, thus permitting the pathologist an accurate prediction of the etiologic agent in clinically undiagnosed or unsuspected viral disease. By correlating viral inclusion identification, clinical history, and physical findings the anatomic pathologist can make the diagnosis of many viral diseases rapidly, inexpensively, and with a high degree of accuracy when compared to the complex, expensive, and time consuming procedures of animal inoculation, tissue culture, or serology.