Use of immunoelectron microscopy to show Ebola virus during the 1989 United States epizootic. A filovirus, serologically related to Ebola virus, was detected by "post-embedment" immunoelectron microscopical examination of MA-104 cells. These had been infected by inoculation with serum samples obtained during the 1989 epizootic in cynomolgus monkeys (Macaca fascicularis), imported from the Philippines and maintained at Reston, Virginia, USA, a primate holding facility. The immunoelectron microscopy method, when used in conjunction with standard transmission electron microscopy (TEM) of infected cells, provided consistent results and was simple to perform in this epizootic. It is concluded that immunoelectron microscopy is potentially useful in the direct immunological diagnosis of Ebola and related filoviral infections (such as Marburg) in clinical samples obtained from those with acute infection. 