Postnatal treatment of mice with antibodies against the T cell receptor complex (TcR) prevents the differentiation of mature T cells in the thymic medulla without affecting the generation of most immature cortical thymocytes, thus interfering with a discrete stage of intra-thymic T cell differentiation at the cortex/medulla transition. This result has been interpreted as indicating a direct role of the TcR in the differentiation of immature to mature T cells, possibly via TcR-ligand interactions during direct cell-cell contact. Here we analyze the effect of anti-TcR (V beta 8 family) and anti-CD3 (epsilon chain) antibodies on distinct intra-thymic cell-cell interactions in vivo. We find that the maturation arrest of thymocytes correlates with a nearly complete abrogation of interactions of corresponding immature thymocyte with I-A/E+ cortical epithelial cells and I-A/E+ medullary dendritic cells, while preserving interactions with adherent I-A/E- macrophages. It is proposed that the blockade of thymocyte-epithelial cell recognition in the cortex by anti-TcR antibodies prevents the translocation of thymocytes into the medulla and their subsequent differentiation and selection, including interactions with dendritic cells. Interestingly, the anti-CD3 mAb treatment seems to spare the intra-thymic development of the CD3+, CD4-/CD8- T cell lineage.