Abstract: In the immunoassay of antigens in liquids, a reaction mixture is formed containing the liquid under assay, labelled antigen, and a mixed binding reagent which contains an antigen-binding site and a label-binding site, the two sites being spaced apart in the reagent so that a single molecule of labelled antigen cannot bind to both sites. The label is one whose activity is changed upon binding to a label-binding site, and the amount of antigen in the original liquid sample is determined by measuring the activity of the label in the reaction mixture. A preferred label is a fluorophore. The mixed binding reagent preferably consists of two antibodies linked together.