Abstract: In this paper,we described a Flow Cytometry-additivity test to analyze the monoclonal antib-odies to recognize different antigenic determinants on the tumor cells.The target cells are mixed.with two antibodies either separately or simulteneously,and are stained with FITC-Ig conjugate.The amount of bound antibody is quntitatively measured by the use of Flow Cytometry.AI （ad-ivity index） is determined using a computer program developed for curve fitting of Flow Cyto etry.Using this test,it has been shown that the monoclonal antibodies Lewis ^a,CS₃,can be reco-gnized at the same sites on two tumor cell line used.