Colloidal gold immunocytochemistry was first introduced by Faulk and Taylor (1), and has rapidly become a major technique in electron microscopy, covering many aspects of biological research. The rapid expansion of this technique in electron microscopy is first the result of its simplicity compared with other labeling techniques, and secondly, the result of the properties of the gold probes themselves. These probes are usually 3-15 nm in diameter and coated with immunologically active proteins. They have the advantages of being very electron dense, giving a characteristic appearance that cannot be confused with other biological structures; they are highly sensitive, producing very specific labeling of both monoclonal and polyclonal antibodies; they are also permanent, nonhazardous, and can be easily quantified.