Methods of assay

@ An electrochemical specific-binding assay of a ligand wherein one of the components is redox-modified by having incorporated into its molecular structure a redox centre, and which includes the step of determining the extent to which an electrochemical characteristic of the components is perturbed by complex formation and/or by controlled external influences. Redox centres are molecules, atoms, groups, ions, radicals etc capable of undergoing reduction and oxidation. A preferred example is ferrocene or its derivatives. The electrochemical apparatus will typically contain a working electrode, which is preferably solid. Components may advantageously be immobilised onto such a solid working electrode. When controlled external influences are employed, these may comprise displacement of non-immobilised electroac- tive components relative to the working electrode. The method is particularly applicable to the assay of antigens, haptens and antibodies. The use of redox centres avoids radioactive labels and provides a simple, rapid, sensitive and specific assay method.