An assay method which involves: contacting a sample with a binding partner which binds specifically to a first substrate molecule which may be in the sample but not to a second substrate molecule which may be in the sample; and detecting a response indicative of binding, or of the extent of binding, between the first substrate and the binding partner, characterised in that the binding partner includes a metal ion that is required for binding between the binding partner and the substrate or product and that the response detection is performed without separating the bound first substrate from the unbound first substrate.