Abstract: A method of assaying for a ligand in a sample which includes incubating, simultaneously or in any desired sequence,a) the sampleb) a reagent X andc) a reagent Y immobilised on the surface of an optical structure capable of exhibiting surface plasmon resonance,one of reagents X and Y is a specific binding partner to said ligand and the other of reagents X and Y is either a ligand analogue or a specific binding partner to said ligand, reagent X being such, that any formation of a direct or indirect complex between reagents X and Y results in an optical surface having an appreciably enhanced optical thickness as compared to the optical thickness of the optical surface which would prevail in the absence of reagent X,which method includes the step of determining whether and, if desired, the extent to which and/or rate at which the surface plasmon resonance effect exhibited by the optical structure is altered by said complex formation.