Identification of type C viruses by electron microscopy.

The cores of a small proportion of the particles in purified preparations of type C viruses are characterized by two concentric shells that have a "double-ring" appearance in specimens prepared for electron microscopy by negative staining or thin sectioning techniques. This distinctive feature allows such particles to be recognized as viral in impure preparations. Detectability of double-ring particles is considerably enhanced by treating the specimens with a non-ionic detergent before fixation and negative staining. The limit of detectablity of type C virus (identified by the double-ring feature) in the presence of a large excess of nonviral particulates of similar size and density was assessed.