The complement-mediated lysis of human lymphocytes by three monoclonal anti-human T-cell antibodies OKT3.PAN, OKT4.INd and OKT8.SUP was studied. The percentages of Ficoll-Hypaque-isolated mononuclear cells lysed by these antibodies were respectively: 65% for OKT3.PAN, 39% for OKT4.IND and 20% for OKT8.SUP. Optimal lymphocytotoxic reactions were noticed when unabsorbed rabbit serum was used as the source of complement (C). Addition of heat-inactivated human, mouse and newborn calf sera but not of foetal calf serum inhibited the lytic activity of the antibodies. Treatment of peripheral mononuclear blood cells with OKT3.PAN and C abrogated their mitotic response to PHA and Con-A. Sheep erythrocyte rosetting lymphocytes (E+ cells) treated with OKT4.IND or OKT8.SUP and C exhibited no marked changes in responsiveness to PHA, Con-A or allogeneic non-T cells. However, only E+ cells enriched with OKT4.IND-reactive cells responded to purified protein derivative, proliferated in the autologous mixed lymphocyte reaction and were highly sensitive to hydrocortisone suppression when stimulated by PHA. Our data indicate that these monoclonal antibodies can be regarded as invaluable tools for enumeration, characterization and functional assessment of human T cells and their subclasses.