Abstract: The invention discloses a method for detecting and/or quantifying a hapten in a homogeneous phase, comprising the following steps: adding a known quantity of a hapten inhibitor complex to the solution containing the hapten to be detected and/or quantified; adding to the solution a quantity of antibodies corresponding to the quantity of the hapten/inhibitor complex; adding to the solution a type C &bgr;-lactamase having an active site for two substrates in antigenic competition in the said active site, the first substrate being a reporter substrate capable of being transformed into a detectable and/or quantifiable product, preferably by visible UV radiation measurement, the second substrate being the hapten/inhibitor complex acting on the hydrolysis rate of the reporter substrate; detecting and/or quantifying the concentration of the product resulting from the transformation of the reporter substrate, the Km constant of the reporter substrate being at least a hundred times higher than the Km constant of the ha