Two distinct separation fluoroimmunoassays were developed to determine serum levels of bendazac and its main metabolite 5-hydroxy bendazac for use in early pharmacokinetic studies and for long-term therapeutic drug monitoring should this prove necessary. In both assays fluorescein-labeled analyte was used as tracer and the antibodies were coupled to magnetizable particles. The use of a magnetizable solid phase enabled a simple and rapid separation step and thus a high sample throughput. There was no crossreactivity between the drug and its metabolite in either assay, indicating that highly specific antibodies had been produced. The results from both assays correlated well with serum levels determined by a high performance liquid chromatography method.