SEVERAL recent studies have suggested that interactions between thymocytes and thymic stromal cells are essential for the development and elimination of antigen-reactive T lymphocytes1-4. It is important, therefore, to characterize the stromal cells involved in presentation of antigen in the thymus. In a previous report, we demonstrated, using T-cell hybridomas, that three distinct types of antigen presenting cells in the thymus (cortical epithelial cells, macrophages, and dendritic cells) constitutively expressed self haemoglobin/la complexes5. Here we report that one of these cell types, the cortical epithelial cell, does not induce stimulation of T-lymphocyte clones even though the antigen/la complex required for antigen-specific recognition is present. This lack of response occurs with both TH1 and TH2 clones. Responsiveness of the TH2 clone can be restored by adding the murine lymphokine interleukin-1β to the culture system. © 1989 Nature Publishing Group.