Abstract: A method and apparatus for quantitatively analyzing, in a flowing stream, materials in a biological or chemical sample utilizes immobilized antobodies reversibly precharged with fluorescently labeled antigens. The labeled antigens are competitively displaced by unlabeled antigens in the sample, after the sample antigens have been segregated into zones by a chromatographic device, such as the reverse phase high performance liquid chromatograph. Displaced labeled antigens are measured by a fluorescence detector. A plurality of antibody species may be concurrently utilized, thereby allowing quantification of a plurality of antigens from a single sample.