Abstract: Method of assaying hapten in a sample, in which said sample is placed in contact with antibodies capable of recognizing hapten and with a predetermined quantity of a reagent capable of competing with said hapten-fixing antibodies. The reagent is a conjugate formed by a single-stranded fragment of nucleic acid with a compound capable of being recognized by said hapten-recognizing antibodies. The fixed or unfixed reactant is detected by antibodies capable of recognizing the nucleic acid fragment of said conjugate. Detection of said reactant by anti-fragment antibodies of said nucleic acid enhances, in particular, the sensitivity of the assay.