Two independently produced monoclonal antibodies, UCHT1, an IgG1 antibody, and OKT3, an IgG2 antibody, which specifically identify all human peripheral T lymphocytes, were shown to possess similar functional properties. Both were highly mitogenic to peripheral blood T lymphocytes, and UCHT1 blocked specific T lymphocyte killing of autologous B lymphoblasts when present during either the induction or the cytotoxic phase of the reaction; this, too, is a property reported for OKT3 in an analogous situation. Both antibodies also inhibited the 6-day proliferative response to soluble antigens and mixed lymphocyte culture, although this may be attributable to their strong mitogenicity earlier in culture. These properties were not found with other monoclonal antibodies directed against human T lymphocytes, and suggested that UCHT1 and OKT3 recognize important functional structure(s) on the T cell surface. The identity of the antigen recognized by these two antibodies was demonstrated by 2-directional co-capping, by cold antibody inhibition of binding of radiolabeled antibody, and by immunoprecipitation and two-dimensional gel analysis of labeled T cell surface proteins. Both UCHT1 and OKT3 bound with similar affinities (Ka approximately 2 X 10(9) M-1) to the same number of binding sites (2.8 X 10(4) sites/activated T cell), and both precipitated the same glycoprotein complex with a m.w. of approximately 19,000 in the presence of reducing agent.