OKT3, a monoclonal anti-human T cell antibody (IgG2), was found to induce DNA synthesis in human peripheral lymphocyte cultures. OKT3 induced maximal mitogenesis at a concentration of 10 to 20 ng/ml and was about 20-fold more potent than PHA as a mitogen. No high-dose inhibition of thymidine incorporation was noticed at concentrations up to 2.5 microgram OKT3/ml. The monovalent Fab fragment of OKT3 was also mitogenic but about 100 times less potent than the parent IgG. OKT3 appeared to be a T lymphocyte mitogen as only sheep red blood cell rosetting lymphocytes were responsive. Quantitative studies on the binding of 125I-labeled Fab fragment of OKT3 to human lymphocytes showed an average of 5.1 x 10(4) receptor sites/cell with an association of about 10(8) M-1 at 37 degrees C, with no heterogeneity of the cell binding sites. These data suggest a strong interaction of the monoclonal OKT3 with a limited number of identical T cell membrane receptors. As this interaction can trigger mitogenesis, the cell membrane determinant recognized by OKT3 could be described as a "T cell stimulation receptor." The mitogenecity of the lymphocytes is not solely dependent on cross-linking of these receptors.