A microtitre plate is supplied precoated with Sulphamethoxazole antibody. Antigen present in the standard and sample competes with horseradish peroxidase labelled Sulphamethoxazole (enzyme labelled antigen) 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 addition of acid. This produces a colour change from blue to yellow, and the absorbances are read at 450 nm.