In the indirect ELISA test, the sample antibody is sandwiched between the antigen coated on the plate and an enzyme-labeled, anti-species globulin conjugate. The addition of an enzyme substrate-chromogen reagent causes color to develop. This color is directly proportional to the amount of bound sample antibody. The more antibody present in the sample, the stronger the color development in the test wells. This format of indirect ELISA is suitable for determining total antibody level in samples (Newcastle disease virus, B. abortus, etc.). Detailed information about indirect ELISA application in the determination of antibody titer and procedures of antibody concentration determination are discussed in the following section of ELISA applications.