Particles that can be used are particles of metals and metal compounds such as metal oxides, metal hydroxides, metal salts and polymer cores which are coated with metals or metal compounds (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 in