A method for the qualitative or quantitative determination of an analyte in a test sample, wherein a labelled reagent is caused to be immobilised in bound form on a solid phase to provide an indication of the presence or quantity of the analyte in the sample is characterized in that the labelled reagent comprises a gold sol bound to a substance capable of specifically binding to the analyte or to a specific binding partner therefor, at least 75% by weight of the particles of the gold sol having a mean diameter of less than 5 nanometres.