A previous study from this laboratory described a monoclonal antibody, S511, that reacted with the T cell antigen receptor on a human T cell leukemia and also on 1-2% of circulating T lymphocytes in all normal individuals tested. The data presented in the present study demonstrate that, when normal T lymphocytes are cultured with or without irradiated non-T cells in the presence of soluble S511 antibody, a concentration- and time-dependent proliferation of the S511-reactive population occurred. Proliferation indices as high as 184 times greater than control were observed, which represents a major stimulatory effect on the initially minor S511+ subset. When S511+ cells were studied for evidence of prior activation, they were shown to be unresponsive to interleukin 2 (IL-2) unless exposed to S511 antibody, and were shown to be in the G0/G1 phase of the cell cycle. Thus, the S511 antibody activated resting normal T cells in a manner analogous to specific antigen binding to the T cell antigen receptor. The leukemic S511 molecule has been shown previously to differ from most other antigen receptors in the mobility of the two chains at 43 and 38 kD and the neutral isoelectric point of each chain. Expansion of reactive normal cells by S511-Sepharose permitted the development of IL-2-dependent T cell lines enriched for S511-bearing cells. The antigen receptor molecules on one such polyclonal S511-enriched T cell line were immunoprecipitated with S511 antibody and shown to have comparable mobility to that present on the leukemic cells, but to possess a greater heterogeneity of mobility. Thus, the leukemic cells and normal cells express similar T cell receptor molecules. The differences in the S511 T cell antigen receptor molecule possibly relate to differences in glycosylation or polypeptide structure.