The induction of non-responsiveness in specific clones of T cells in vivo might be expected to reverse immunologically mediated disease processes. With this goal in mind, experiments in vitro with cloned T cells have investigated the mechanisms of induction of anergy. Resting T cells can be functionally inactivated in vitro by high doses of appropriate peptide in either the presence or absence of antigen presenting cells. During the induction of anergy, the modulation of the surface phenotype of T cells is similar to that of cells proliferating in response to an immunogenic stimulus. The amount of T cell receptor at the cell surface is down regulated, whereas the CD2 and CD25 receptors are increased in density. In contrast, CD28 has been identified as a membrane protein that is differentially regulated during activation and the induction of non-responsiveness. Ligation of CD28 provides an efficient costimulatory signal for activation of T cells. In this report, we show that not only resting cells, but also fully activated T cells can be rendered non-responsive and that this process is accompanied by profound downregulation of CD28, both at the level of cytoplasmic mRNA and surface expression of the mature protein. This observation anticipates clinical intervention in immunologically mediated disease, where the target T cells are more likely to be activated than in a resting state.