The microtiter plate provided in this kit has been pre-coated with rabbit IgA. Standards or samples are then added to the appropriate microtiter plate wells with Horseradish Peroxidase (HRP) -conjugated antibody preparation specific for rabbit IgA, mix well and incubated. The more the amount of rabbit IgA in samples, the less HRP-antibody bound by pre-coated rabbit IgA. Then a TMB (3,3',5,5' tetramethyl-benzidine) substrate solution is added to each well. And the color develops in opposite to th