Chemically functionalized metal cluster compounds have demonstrated important advantages over colloidal gold as biological microscopy labels. They are covalently cross-linked to the targeting biomolecule, and therefore may be conjugated to a wide range of molecules which cannot be labeled with colloidal gold. The 1.4 nm Nanogold® cluster has been conjugated to peptides, lipids and oligonucleotides, some of which have been proposed as elements of novel molecular wires and novel materials. Dissociation of colloidal gold particles from the conjugate probe, a source of error in quantitative immunogold studies, is greatly reduced by covalent cross-linking. Nanogold® is an uncharged molecule, and because its surface is completely coordinated by organic ligands, nonspecific binding is greatly reduced. Nanogold® conjugates also show greatly enhanced penetration into cells and tissue sections. However, gold probes larger than Nanogold® are desirable for improved visualization in specimens with electron-dense regions or staining, or for applications such as double labeling studies with different sized gold labels, or visualizing wider antigen distributions.