Imazethapyr is a herbicide belonging to the imidazolinone class of compounds. It is the active ingredient of a commercial herbicide PURSUITe. Polyclonal antibodies have been prepared in rabbits and sheep which specifically recognize this class of imidazolinone compounds. Using the immune rabbit serum, an enzyme-linked immunosorbent assay (ELISA) has been developed for the entire class of commercial imidazolinone herbicides including imazaquin, imazapyr, imazethapyr and imazamethabenz methyl. The quantitation of imidazolinones in soil requires a certain amount of sample pretreatment, thus the through-put is not ideal. A simpler immunoassay method for screening large amount of soil samples economically would be useful. Taking the sheep polyclonal immune serum, we used a fluorescent immunoassay employing optical fiber and fluorescence (US Patent 4,582,809, 19861, to assay for imazethapyr. Purified sheep antibody was immobilized on quartz fibers. A mixture of fluorescein-labelled imazethapyr analog and free imazethapyr was presented to the fiber for direct competition of the antibody binding sites or displacement of a previously fluorescein labelled fiber. The response time for the detection of imazethapyr ranged from seconds to minutes. The sensitivity of the assay was 1 nM. This binding of the fluorochrome to the fiber was reversible by washing with a phosphate buffered saline. Multiple measurements were easily processed with a single fiber over the course of several hours. Analysis of imazethapyr residue in soil can be accomplished by subjecting a clarified soil extract solution directly for analysis without further treatment. The crossreactivity data indicates that the assay is apparently specific for the imidazolinone class of compounds.