Abstract: A method and device for performing comparative assay of binding and bindable substances such as antibodies and antigens. The device includes a circulation conduit for continuously conveying a carrier fluid between a support zone therein and a detection zone therein. A competitive binding substance is fixed in the support zone and is capable of competitively binding a test substance and an analytical detection substance such that changes in concentration of the test substance in the carrier fluid responsively cause changes of concentration of analytical detection substance in the carrier fluid. Each of the analytical detection substance (antigen-fluorochrome, for example) and the test substance (antigen, for example) are bound to the binding substance (monoclonal antibody, for example) in proportion to the concentration of the test substance in the carrier fluid.