Thymic lymphocytes from normal inbred Lewis/Wistar rats were cocultured with syngeneic Sertoli cell-peritubular cell preparations in the presence of heterologous or allogeneic serum. Thymic cells cultured in this manner bound to Sertoli cells, became autosensitized , and markedly altered syngeneic Sertoli cell surface properties and remodeling functions in vitro. In contrast, control thymic cells incubated with Sertoli cells in autologous or syngeneic serum did not become sensitized. Coculture of autosensitized thymic cells with syngeneic seminiferous tubule segments, or local transfer of such lymphocytes into syngeneic rat testes, resulted in intratubular infiltration by "light cells." Intratesticular injection of autosensitized thymic cells was followed by derangement of the seminiferous epithelium, and by morphologic changes characteristic of experimental autoimmune orchitis. Thymic cells incubated with Sertoli cells in autologous or syngeneic serum did not elicit these changes. Thymic cells incubated with peritubular cells in heterologous or autologous serum behaved like control thymocytes, and were not sensitized. Data presented indicate that thymic cells are potentially capable of recognizing syngeneic Sertoli cell self-antigens. We speculate that factors normally present in serum may inhibit the recognition by thymic lymphocytes of antigenic determinants present on Sertoli cells. We discuss the possibility that the modulation of interactions between immature thymic lymphocytes and Sertoli cells is implicated in the prevention of autoimmune reactions against the testis.