Reagent for scintillation proximity assays

The invention relates to scintillation proximity assays, in vitro and in cells in multiwell plates or in other formats. The invention, provides a support in which or upon which there is immobilised a system to be studied and wherein the support comprises a phosphor. In one aspect the system comprises one member of a specific binding pair immobilised in or onto the support. In another aspects the support is treated to enable the attachment and/or the growth of cells. The phosphor is capable of absorbing energy in an integrated way from radioactive emissions from a radiolabelled reagent when it is bound to the surface of the support. Upon activation by means of orange, red or infra red radiation, the stored energy is released as light which may be detected using a charge coupled device.