Abstract: The method of measuring analytes in biological fluids is disclosed wherein a specific binder to a given analyte is covalently immobilized onto a solid support to which a labeled analyte is pre-reacted and stabilized to form a binder-labeled analyte complex. A sample is contacted with said immobilized complex wherein an analyte in the sample, if present, competes with the labeled analyte bound to the immobilized binder for binding sites on said binder thus displacing a given amount of the labeled analyte which is directly proportional to the amount of analyte present in the sample. The affinity of the labeled analyte to the analyte's specific binder is lower than the affinity of the unlabeled analyte to the same binder.