Each microtitre plate is supplied precoated with a SEM antibody. SEM (antigen) in the Nitrofuran standard (multi antigen) and, if present, in the sample competes with the horseradish peroxidase labelled SEM in the Nitrofuran conjugate for a limited number of antibody sites on the microtitre plate. After incubation at room temperature, to allow a competition reaction to take place, the microtitre plate is washed to remove excess reagents. The enzyme substrate is added. After an incubation period to allow maximum colour development, the colour reaction is stopped by the addition of acid. This produces a colour change from blue to yellow, and the absorbances are read at 450 nm. A standard curve is then constructed to determine the SEM concentration in the sample.