Abstract: An apparatus and method for performing enzyme-linked immunological sandwich assays (ELISA) by affinity chromatography. Formation of the antibody-antigen-conjugate complex, and the development of a chromatophore indicator take place in a semi-automated, walk-away, fashion. The immunoassay apparatus comprises a chromatography tank, which further comprises a reaction compartment and a plurality of liquid solvent reservoirs, with a first-liquid reservoir being configured higher than a second-liquid reservoir. The apparatus allows automatic sequential and discrete movement of wash solution, followed by a color development solution, with unbound conjugate being substantially washed away from the immunological sandwich complex before the color development solution automatically comes into contact with the immunological complex.