Thin section immuno-electron microscopy has been successfully applied to investigate and identify the classical and mild form of HFRS viruses isolated in the People's Republic of China. The results showed that all the 8 strains studied (derived from different parts of China, adapted in different cell lines) share a common morphology and morphogenesis. Essentially, the viruses possess characteristics of members of the Bunyaviridae Family, however, differing by a larger size and size variation and formation of cytoplasmic viral inclusions.