The present invention relates to microspheres comprising protein signal precursor molecules, or a carrier protein bonded to signal precursor molecules, wherein said signal precursor molecules are activatable to generate a detectable signal whilst remaining bonded to the carrier protein. Also disclosed is a method of making such microspheres comprising the steps of mixing protein molecules with a matrix former in solution adding a reducing reagent to the mixture removing the reducing reagent and removing the matrix former to leave microspheres of protein molecules. Also disclosed are bioassay methods using the microspheres to provide signal amplification, including an amplification cycling procedure.
