Antigens present during neonatal life are recognized as self and individuals are tolerant to these antigens 1 . In normal individuals T cells are tolerant to most self proteins but we still know little of the mechanism(s) by which tolerance is established. A requisite part of the current negative selection model of self tolerance is the expression of self proteins complexed with major histocompati-bility complex molecules in the thymus 2,3 . As MHC proteins bind antigens and present them to the receptor on the antigen-specific T cell, then for tolerance to self to occur, it is possible that each self protein must be processed and presented by an MHC molecule. As a result of the development of a unique T-cell hybrid reactive to the self protein murine haemoglobin, we have shown that in normal animals this self protein is continuously processed and potentially presented in an MHC-restricted manner 4 . Here we show that self haemoglobin is being processed and presented by thymic antigen-presenting cells as early as gestational day 14. We also demonstrate that three types of thymic stromal cells, namely macrophages, dendritic cells and cortical epithelial cells, can present the haemoglobin self antigen in vivo. This surprising presentation of a self antigen by thymic cortical epithelial cells implies that they could be involved in T-cell development and negative selection. © 1989 Nature Publishing Group.