The mycotoxin deoxynivalenol (DON) is produced by the mold Fusarium graminearum and is found worldwide on cereal grains, in particular wheat and maize. Each year this compound, also known as 'vomitoxin' causes substantial losses to agricultural productivity. Three monoclonal antibodies were developed following the immunization of mice with a conjugate of DON and ovalbumin. One of these antibodies, produced by clone #22, was selected for the development of a competitive direct ELISA (CD-ELISA). This format consists of competition between a DON horseradish peroxidase conjugate (DON-HRP) and free DON for antibody attached to microwell plates. Color development in the assay was inhibited 50% (IC50) by 18 ng DON/ml in phosphate-buffered saline (PBS). The antibody from this clone showed strong cross-reactivity to 3-acetyl deoxynivalenol (3-Ac-DON), with an IC50of 2.9 ng ml−1Cross-reactivity to 19 other trichothecene mycotoxins was low. The CD-ELISA was applied to wheat spiked with DON over the range 0.01-10 w g/g and extracted with a 10-fold excess of PBS. The midpoint for color development in the assay using this extraction was 0.27 w g DON/g wheat. Recoveries over the range 0.05-5 w g/g averaged 88.7% with a coefficient of variation of 10.9%. This assay is sufficiently sensitive and rapid to permit the screening of DON in wheat below the US Food and Drug Administration advisory level of 1 ppm in human food.