Interdigitating (IDC) cells of the thymus have been characterizedin situby their ultrastructure and phenotype. Thymic dendritic cells (DC), thought to represent theirin vitrocorrelate, resemble splenic DC in their ability to initiate peripheral T cell responses.In vivo, however, DC of the thymus have been implicated in tolerance induction, although at one time they were thought to impart MHC-restriction on developing T cells. Our present understanding of these areas is reviewed here.Anin vitromodel has been developed to address directly the function of DC in the thymus. Mature DC and immature thymocytes migrate into deoxyguanosine-treated thymus lobes where they adopt a reciprocal distribution, DC homing primarily to the medulla while the thymocytes remain in the cortex. These observations support the close relationship between thymic DC and IDC and provide a powerful tool to examine the role of DC in thymocyte ontogeny.