Four anti-T lymphocyte monoclonal antibodies were produced by the hybridoma technique. Percentages of miscellaneous labelled cell suspensions, as profiles of fluorescent histograms, showed a reactivity of these 4 reagents quite comparable to that observed with anti-CD3 monoclonal antibodies. Labelling summation studies, cell-sorting studies and blocking experiments gave similar results as gave other anti-CD3 reagents. Biochemical study and functional tests: modulation effects and mitogenic properties, argued for the belonging of these 4 antibodies to the CD3 cluster. The actual data concerning the CD3 molecule are briefly resumed to underline the interest of such antibodies in the understanding of T-lymphocyte activation mechanisms during the immune response.