The invention relates to a reagent for carrying out assays in accordance with the FRET-principle, containing an acceptor dye as well as particles exhibiting a donor dye, whereby the donor dye and the acceptor dye are located in different chemical phases, the acceptor dye is provided as bound to the particles and the particles are essentially impermeable to the sample medium or to components of the sample medium affecting the luminescence characteristics of the donor dye. The invention furthermore relates to a method for the qualitative and/or quantitative determination of at least one analyte and/or component of a liquid measuring medium, comprising the addition of the reagent according to the invention to the measuring medium.