This ELISA assay utilises the principle of competitive binding to measure the concentration of hormone in plant extracts.

The ABA (abscisic acid) hormone has been pre-coated on the surface of the reaction wells. The plant extract sample, containing an unknown amount of hormone, or standards are mixed in the reaction well with a known amount of antibody to ABA. During incubation, the competitive inhibition reaction occurs between pre-coated ABA and hormone in the samples with the ABA-specific antibody.