Competitive ELISA analysis of Clenbuterol / Salbutamol/ Ractopamine was performed by coating wells of a 96-well plate with 50 µl per well of Clenbuterol-BSA diluted in carbonate/bicarbonate buffer, at a concentration of 1 µg/mL overnight at 4℃. Wells of the plate were washed, blocked with 1%BSA blocking buffer, and incubated with 100 µl per well of Clenbuterol monoclonal antibody at concentration of 1 µg/mL with serial diluted Clenbuterol/Salbutamol/Ractopamine starting from a concentration of 10ug/ml for 1 hours at room temperature. The plate was washed and incubated with 50 µl per well of an HRP-conjugated goat anti-Rabbit IgG secondary antibody at a dilution of 1: 5,000 for one hour at room temperature. Detection was performed using an Ultra TMB Substrate for 10 minutes at room temperature in the dark. The reaction was stopped with sulfuric acid and absorbances were read on a spectrophotometer at 450 nm.