Although the electron microscope has been used steadily by our laboratory, its peak activity occurred about two decades ago when it was the vehicle for: (i) the discovery of the first important gastroenteritis virus, the Norwalk virus, followed in succession by the discovery of the Hawaii virus and Montgomery County virus, other agents of viral gastroenteritis; (ii) the discovery of the hepatitis A virus; (iii) the initial detection in the United States of human rotavirus (which was discovered in Australia), the most important cause of severe diarrhea of infants and young children; and (iv) understanding the natural history and importance of rotaviruses in infants and young children. In the past decade it was also instrumental in studies of hepatitis E virus. This past year marked a striking resurgence of activity involving the electron microscope because it proved to be: (i) the only method for detecting recombinant Norwalk, Desert Shield, Hawaii and Toronto virus-like particles that were expressed in insect cells; and (ii) the only method to demonstrate specific antigenic relationships among these recombinants (by immune electron microscopy). As a result, the electron microscope has been an important adjunct to many of the projects of the section providing seminal information in the progress of molecular biologic (as well as other) studies. The scope of the use of the electron microscope is evidenced by the observation that over 100 individual experiments were conducted by electron microscopy since the previous annual report.