Peripheral T cells and cells from the thymic medulla have many phenotypic characteristics in common, and both populations are quite distinct from cortical thymocytes. The cells that migrate out of the thymus (thymus migrants) may be detected in the periphery as fluorescent cells shortly after intrathymic injection of fluorescein isothiocyanate. Our previous work has established that they resemble T cells and medullary thymocytes rather than cortical thymocytes, and are fully functional. In this report, we consider several phenotypic characteristics that differ between medullary thymocytes and peripheral T cells, namely size, buoyant density, and sensitivity to a new monoclonal antibody (B2A2) and complement. By these criteria, the fluorescence-labeled thymus migrants in spleen and lymph node tend to resemble medullary thymocytes and are distinct from the majority of T cells that surround them. This suggests, but does not prove, that migrants originate in the medulla. After adult thymectomy, the small population of medullary-thymocyte-like cells disappears from spleen and lymph node, further supporting the idea that cells leave the thymus with a medulla-like phenotype, and acquire normal peripheral T cell phenotype only after their arrival in the periphery. Thus, although thymus migrants appear to leave the thymus in an immunocompetent state and are phenotypically mature by most criteria, there are a few final maturation steps that occur after arrival in the periphery.