The advantages of the ELISA are similar to other antibody-labeled reactions which include specificity, sensitivity, inexpensiveness, and safety. Since the enzyme label is the critical portion of ELISA, its selection is very important. The enzyme selected should be stable under the conditions used for storage, cross-linking, and the assay. The most effective enzymes will have a high specific activity and will be inexpensive. Equally important is that the enzyme must be absent from the antigen or the antiserum preparations used in the serological tests, otherwise false positive tests would result. False negative results could stem from the presence of enzyme inhibitors or inactivators in the serological reagents. Appropriate controls must be incorporates into the test to identify these potential problems in ELISA. The enzyme catalyzes a change in a substrate molecule, but the enzyme itself is not consumed in the process. The enzyme molecule continues to act on more substrate molecules and produce thousands of colored products. Thus, in ELISA, antigens are detectable when present in only nanogram and picogram quantities. One form of the ELISA is carried out in a microtiter plate which is used as the solid phase for adsorption of antigen （Figure 1）. Albumin or powdered milk is used to block the remaining sites which could result in false positives. Monoclonal antibodies will react with the antigen attached to the microtiter plate. The complex antigen-antibody is then reacted with enzyme conjugates to a goat antimouse IgG, which catalyzes the substrate p-nitrophenyl phosphate, to form a colored product. Excess reagents are removed by washing, and enzyme activity can be measured with an ELISA plate reader at 415nm. The amount of colored product is an indirect expression of the monoclonal antibody amount in the sample.