T-cell colony-forming cells (CFC), the cells from which T-lymphocyte colonies are initiated when cells are grown in semisolid medium, have been studied. CFC have previously been shown to be more frequent in populations of rather mature thymic cells and peripheral lymphocytes than in populations of immature cortical thymocytes. We have here evaluated whether CFC in spleen and lymph nodes are confined either to newly emigrated thymocytes or to older recirculating T cells. It is shown that CFC in spleen have the same Lyt phenotype as thymic and lymph node CFC, namely Lyt-1+2+, and that the peripheral complement of CFC is rather slowly built up after birth or after total body irradiation and bone marrow reconstitution. Results obtained after fluorescence-activated cell sorting, hydrocortisone injection, or thymectomy indicate that CFC are present in a broad variety of peripheral T-cell populations in accordance with age and thus that T-cell colonies can be used to test the proliferative capacity of the 'average' peripheral T cells of the Lyt-1+2+ phenotype.