Two sensitive methods based on a monoclonal anti-2,4-dichlorophenoxyacetic acid (2,4-D) antibody, a flow injection immunoanalysis (FIIA), and a fiber optic immunosensor are described and evaluated for the quantitative determination of 2,4-D in water samples. Detection limits for 2,4-D of ∼0.03 μg/L with FIIA and 0.2 μg/L with the fiber optic immunosensor can be achieved, i.e. well below the maximum concentration permitted, in the case of FIIA, by the EC Drinking Water Directives of 0.1 μg/L. The midpoints of the 2,4-D test are found at concentrations of 0.3 μg/L with FIIA and 10 μg/L with the fiber optic immunosensor. Measurable concentrations range from 0.03 to 3 μg/L with FIIA and from 0.2 to 100 μg/L with the fiber optic immunosensor. The FIIA did not require concentration or cleanup steps for ground and surface water samples. In addition, validation experiments with GC/MS showed a good correlation of the data for the FIIA system. However, both tests can be applied as alarm systems for monitoring poten...