A new class of gold immunoprobes has been developed with substantially different characteristics from the usual colloidal gold probes. Instead of chunks of gold metal (colloidal gold) just adsorbed to immunoglobulins by ionic or hydrophobic interactions, well-defined gold compounds are covalently attached to antibodies. Gold, along with several other metals, is known to form organo-cluster compounds with multiple metal atoms. The first gold cluster immunoprobe developed was undecagold (Au11), containing 11 gold atoms in an 0.8-nm sphere (Figure 1), covalently attached to Fab′ antibody fragments (Hainfeld, 1987). A more recent development has been the synthesis of a larger 1.4-nm gold cluster (Hainfeld et al, 1991; Hainfeld and Furuya, 1992).