Human T cell clones (TCC) are antigen-presenting cells (APC) able to present peptides and superantigens (SAg) and to process and present intact proteins. TCC express major histocompatibility complex (MHC) class II antigens and molecules involved in the accessory signal delivery, such as B7.1 and B7.2/B70. Notwithstanding these observations, the role of professional APC has been often denied to T cells because anergy of responder T cells rather than proliferation has been observed following the TCC presentation in the absence of added professional APC. Here, we show that upon stimulation with free SAg. TCC undergo proliferative responses followed, after a 1-week culture, by an SAg-dependent unresponsiveness to T cell receptor (TCR)-mediated stimuli, but not to interleukin-2. The anergy induced by the SAg can not be prevented by the addition of autologous Epstein-Barr virus (EBV)-transformed B cells, indicating that the induction of anergy occurs also in the presence of conventional APC. Conversely, if the TCC are stimulated by SAg-prepulsed irradiated APC, either EBV and TCC, the induction of anergy is not observed. After a 1-week culture. in fact, TCC stimulated with APC-bound SAg responded to TCR-mediated stimuli, irrespective of the APC (EBV or TCC) used for the SAg presentation. Stimulation of TCC with free SAg in a semisolid medium that prevents T-T cell contacts resulted in an activation followed by a state of anergy, suggesting that anergy is the consequence of SAg recognition at the single T cell level. These data indicate that the anergy observed in TCC upon a 1-week culture in the presence of soluble SAg is not the result of an inherent inability of TCC to act as professional APC. Rather the phenomenon depends on the presence of soluble SAg, leading to T cell autostimulation.