Certain substances in the sample may increase or decrease the reaction between the enzyme and substrate in ELISA assays. During a survey of T‐2 trichothecene in food and animal feed 75% of milled grain samples gave a higher O.D. value in competitive T‐2 toxin ELISA than the negative control. In samples spiked with small quantities (10 /tg/kg) of T‐2 toxin this type of reaction resulted in underestimates of toxin content. However, the effect was weak and, owing to the high sensitivity of the assay, it did not result in false negative reactions. The low efficiency of the carrier solvent and natural peroxidases in food and feed were considered to be the cause of the inaccurate reactions. A few fermented and processed foodstuffs and feed gave positive results in the T‐2 toxin ELISA assay, but verification of the results by gas chromatography (GC) showed that the reactions were false. Certain substances in the samples destroyed or decreased the enzyme activity. False positive reactions can be distinguished from correct ones by retesting the extracts in different dilutions.