venient. For 2,4-dichlorophenoxyacetic acid (2,4-D), 3 a An enzyme-linked immunosorbent assay-based disprototype herbicide of the phenoxy acids group, several placement assay was developed for the determination methods such as radioimmunoassays (RIA) (2), enof the herbicide 2,4-dichlorophenoxyacetic acid (2,4zyme-linked immunosorbent assays (ELISA) (3-6), D). Advantage was taken of the cross-reactivity of a flow injection immunoanalysis (FIIA) (7, 8), a fibermonoclonal anti-2,4-D antibody toward 2-methyl-4optic immunosensor (8), CCD camera-based chemiluchlorophenoxyacetic acid (MCPA). MCPA was conjuminescent ELISA ( 9), as well as some amperometric gated with bovine serum albumin (BSA), immobilized immunosensors (10-13) for the detection of this herbion the surface of a microtiter plate, and saturated with cide in water have been reported. In general, for sensthe anti-2,4-D antibody. Due to the low affinity of the ing purposes two major properties of the antibody-anantibody toward MCPA (cross-reactivity of approxitigen interaction are exploited, i.e., the high-affinity mately 30%), the addition of 2,4-D resulted in a disconstant and the low cross-reactivity. Whereas the first placement of the antibody. Remaining antibodies were allows for highly sensitive determinations of up to 10 018 subsequently detected using a peroxidase-labeled goat M antigen (14), the latter is responsible for the selectivanti-mouse antibody. The detection limit was as low ity of an immuno probe. However, these properties of as 0.1 mg/liter for 2,4-D, which complies with the Euroantigen-antibody reactions are not optimal for the depean Union Drinking Water Directives. When 2,4-Dvelopment of immunoassay based on a replacement for-BSA was used instead of MCPA-BSA conjugates, no mat. Development of a sensitive displacement assay significant displacement of bound antibody was obrequires that very small amounts of soluble analyte served.