The maximal T-cell response to its antigen requires presentation of the antigen by a major histocompatibility complex class II molecule as well as the delivery of one or more costimulatory signals provided by the antigen-presenting cell (APC). Although a number of candidate molecules have been identified that are capable of delivering a costimulatory signal, increasing evidence suggests that one such critical pathway involves the interaction of the T-cell surface antigen CD28 with its ligand B7, expressed on APCs. In view of the number of potential costimulatory molecules that might be expressed on the cell surface of APCs, artificial APCs were constructed by stable transfection of NIH 3T3 cells with HLA-DR7, B7, or both. Here, we show that in a human antigen-specific model system, when tetanus toxoid peptide antigen is presented by cells cotransfected with HLA-DR7 and B7, optimal T-cell proliferation and interleukin 2 production result. In contrast, antigen presentation, in the absence of B7 costimulation, results in T-cell clonal anergy. These results demonstrate that it is possible to induce antigen-specific clonal tolerance in human T cells that have been previously sensitized to antigen. The artificial antigen-presenting system provides a useful model for the investigation of the biochemical events involved in the generation of tolerance and for the study of signals necessary to overcome tolerance.