It is generally accepted that thymic epithelial cells (TEC) act as accessory cells in positive selection of pre-T cells. However, our knowledge of the antigen presentation and accessory cell function to human TEC is limited. Here we present results obtained by the use of serum-free cultured human TEC, showing that IFN-gamma-treated TEC are able to support T-cell-mediated responses to the bacterial superantigens (Sag) SEA and SEB, even at very low Sag concentrations. T-cell responses to TEC-presented Sags were dependent on the presence of the adhesion molecules ICAM-1, ICAM-2, LFA-1, and LFA-3, but not on CD4 and CD8 molecules. There is a low but significant expression of B7 molecules on human TEC, and treatment of TEC with anti-B7.1 and anti-B7.2 antibodies before Sag pulsing leads to decreased Sag responses, indicating a significant importance of B7 molecules on TEC. Both CD4+ T-cell lines and CD4+ as well as CD8+ subpopulations of thymocytes showed significant responses, whereas nonseparated thymocytes, CD4+8+, and CD4-CD8- thymocytes did not respond or showed very low responses. In conclusion, the present results demonstrate that cultured human TEC are able to present Sag to thymocytes.