Since colloidal gold was first introduced by Faulk and Taylor [24] as a marker for immunoelectron microscopy, it has been widely used in cell biology [78] and microbiology [4]. This short review is intended to illustrate how it has been applied to problems posed by animal and plant virology and also indicate the advantages and limitations of various labelling methods. Detailed technical procedures are not given but the reader is referred to the original publications.Until the advent of colloidal gold, electron microscopists relied mainly on ferritin and peroxidase as markers for immunocytochemicaI studies. Although these two reagents have proved valuable, colloidal gold offers a number of advantages and only a few limitations. Its extreme electron density renders it highly conspicuous when viewed in the electron microscope. This enables grids to be scanned quickly for labelled areas. Ferritin, on the other hand, is significantly less dense and although it is not too difficult to detect on the cell surface, visualization of ferritin labelled cytoplasmic structures can be a problem. Similarly detection of peroxidase conjugates :may prove difficult, particularly when only low levels of antigen are present. Under these circumstances little DAB reaction product is deposited around the antigenic sites and this necessitates omitting staining with lead and uranyl salts in order to obtain sufficient contrast for visualization. A further drawback of peroxidase is that its detection depends on an enzymic reaction whose product may diffuse away from the antigenic site thus making precise localization difficult [22].