Abstract: A comparison of different types of immunoreactors for a flow injection immunoanalysis (FIIA) system is presented. The use of the FIIA system was demonstrated here with the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D). The hardware system centers around the immunoreactor. In the preferred reactor set-up, a column is filled with protein G-coated porous beads serving as the affinity matrix for the antibody. In the 2,4-D assay a 2,4-D-alkaline phosphatase conjugate competes with the analyte in the sample for binding to the anti-2,4-D antibody. Subsequently p-aminophenol, generated from the enzymatic reaction with p-aminophenyl phosphate, is detected at±150 mV at a carbon working electrode versus Ag/AgCl. The immunoreactor allows for more than 150 measurements. The fully automated assay procedure needs 15 min only for one measurement. For 2,4-D the working range for the determination in water samples was between 1 and 1000 µg/L. Results obtained with the packed column reactor are compared to glassy capillary reactors and miniaturized silicon chip reactors used in previous studies.