The use of colloidal gold particles is now well established in the field of biological affinity labeling. Although the application of colloidal gold probes in microscopy has developed rapidly during the last 30 yr, scientists have been interested in colloidal metal phenomena for much longer. As far back as 1857 (1), Michael Faraday, in an address to the Royal Society, recognized the reactive properties of colloidal metal particles (in particular gold) with light. Indeed, the literature shows that recognition of the potential for use of colloidal gold in microscopy goes back to the 17th century (2). Since 1933 (3), a range of methods has been devised for producing gold sols. It was not until 1971, however, that Faulk and Taylor (4) published a method for binding proteins to gold particles for use in im-munocytochemistry at the electron microscope level. Nowadays, colloidal gold is incorporated into probes that are widely used in the biological sciences both for light and electron microscopy. A gold probe is an electron-dense sphere of gold coated with an immuno-logically active protein. The properties of gold-labeled probes that make them so suitable for light and electron microscopy studies are summarized in Table 1. Although high-quality colloidal gold reagents are available from a range of international manufacturers, the preparation of gold-labeled probes is a relatively straightforward procedure for workers to perform in their own laboratories. Two aspects are to be considered when making the probe, namely, gold sol production and binding the gold to the probe molecule. Such probes may be readily prepared in most well-equipped immu-nochemistry laboratories using combinations of the following approaches (see Notes 1-4).

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