"Particles composed of metals and metal compounds such as metal oxides, metal hydroxides, metal salts and polymer cores which are coated with metals or metal compounds can be used as particles (cf. e.g. U.S. Pat No. 4,313,734; Leuvering et al., J. Immunoassay 1 (1980), 77-91; Leuvering Dissertation (1984), Sol Particle Immunoassay (SPIA): The Use of Antibody Coated Particles as Labelled Antibodies" . . . . . . . . .