The monoclonal antibody OKT11A, which reacts against the receptor for sheep erythrocytes (E receptor) on human T lymphocytes, strongly inhibited proliferation of T cells induced by antigens (PPD and autologous mixed lymphocyte reaction) and by mitogens (concanavalin A (Con A) and OKT3 monoclonal antibody) when added at the start, but not 48 hr after initiation, of culture. The suppressive activity became detectable 36 to 48 hr after the initiation of culture and was maintained throughout the culture periods. In addition, OKT11A inhibited pokeweed mitogen but no Epstein Barr virus-stimulated Ig production of circulating mononuclear cells. OKT11A prevented T cells from acquiring sensitivity to interleukin 2(IL 2) when stimulated by antigens or mitogens. This antibody did not, however, affect the IL 2-dependent proliferation of already activated IL 2-reactive T cells. Moreover, OKT11A strongly suppressed the production of IL 2 in both antigen- and mitogen-stimulated cell cultures. Thus, OKT11A abrogated proliferation of T cells by two distinct but related mechanisms, namely, preventing T cells from becoming sensitive to IL 2 and inhibiting the production of the T cell growth factor. It was also found that OKT11A inhibited proliferation of T cells stimulated by the Ca2+ ionophore A23187, but not by the tumor promoter phorbol myristate acetate (PMA). Mononuclear cells preincubated with OKT11A (in the absence of C) did not suppress proliferation of T cells stimulated by either antigens or mitogens. In addition, OKT11A strongly inhibited the generation of suppressor cells induced by Con A. Neither ConA nor OKT3 antibody inhibited binding of OKT11A to T cells when assessed by indirect immunofluorescence microscopy. Furthermore, OKT11A did not bind to trypsinized T cells, whereas OKT3 antibody did. Finally, OKT3 antibody activated trypsinized T cells to DNA synthesis, and this response was not inhibited by OKT11A. Taken collectively, these results and the findings that PMA bypasses the need to raise cytoplasmicfree calcium((Ca2)i), by directly stimulating normally Ca2+-dependent processes that lead to DNA synthesis of T cells, we conclude the following: 1) The interaction of OKT11A with E receptors results in an event that turns off the process by which Ca2+ gets into T lymphocytes when stimulated by antigens or mitogens. Once (Ca2+)i has risen, this interaction has no effect on ulterior intracellular processes necessary for proliferation of T cells; 2) OKT11A acts neither by blocking activation receptors on T cells nor by inducing the development of suppressor cells. In addition, trivial explanations such as that OKT11A agglutinates T lymphocytes and thereby inhibits necessary mobility for activation of these cells or that this antibody acted nonspecifically or caused toxic effects on T and B lymphocytes were excluded; and 3) We propose that the structure detected by OKT11A (E receptor) functions as a “negative signal receptor” of human T lymphocytes.