The CD3 (HIT3a) antibody recognizes the 17-19 kD ε-chain of CD3 within the CD3 antigen/T cell antigen receptor (TCR) complex. The CD3 antigen is expressed in the cell cytoplasm during the early stage of T cell development and is expressed on the cell membrane at the late stage. CD3 antigen is displayed on 60-80% of normal peripheral blood lymphocytes and 60-70% of thymocytes and plays an important role in signal transduction after antigen recognition by TCR. The clone HIT3a has a strong mitogenic effect at ng level on T lymphocyte proliferation in soluble or immobilized conditions and has an immunosuppressive effect at high dose. Conversely, the clone HIT3b has a strong mitogenic effect on T lymphocytes proliferation only under immobilized condition. NK cells also express the CD3 antigen in the cytoplasm, making the CD3 (HIT3a) antibody a helpful tool for analyzing the development relationship and the common precursor of both T cells and NK cells.