Out of a panel of seven monoclonal antibodies with affinity for human lymphoid cells, three were shown to prevent cytotoxic T cell activity, whereas none affected natural killer cell activity when applied without complement. Anti-OKT3 and anti-Leu-2a, with affinity for all T cells and the cytotoxic/suppressive subset, respectively were both shown to inhibit T killing by their interaction with the effector cell. For anti-OKT3, the inhibition remained after free antibody was washed away. Anti-Leu-2a, in contrast, induced a rapidly reversible inhibition. Using a single cell assay, anti-OKT3 was shown to reduce the lytic ability without affecting target cell binding, whereas anti-Leu-2a prevented the effectors from binding target cells.