Method of quickly detecting and/or assaying antigen by fluorescence correlation spectrometry

It is intended to provide a detection and/or assay method whereby an abnormal prion or an antigen, for example, an antigenic protein such as a harmful protein contained in a food material can be quickly and accurately detected and/or assayed by a convenient procedure. In detecting or assaying an antigen molecule by using the fluorescence correlation spectrometry (FCS), a fluorescence-labeled antibody fragment and a non-fluorescence-labeled complete antibody capable of binding to the fluorescence-labeled antibody fragment via an antigen are employed. Thus, a significant difference in diffusion speed arises between the fluorescence-labeled antibody fragment not bonded to the antigen and a complex formed by the antigen/antibody reaction among the fluorescence-labeled antibody fragment, the antigen and the non-fluorescence-labeled complete antibody. According to this method, an antigen can be detected and assayed by using FCS, even in the case of an antigen with a relatively low molecular weight such as an antigenic protein, independently from the shape or molecular weight of the antigen and thus antigens over a wide scope can be quickly assayed.