Adult thymectomized and lethally irradiated (ATxXB) BALB/c mice, reconstituted with either nude or normal bone marrow (BM) cells, have been studied to evaluate numbers and mitogen responsiveness of T cells and T-cell progenitors in the BM and the spleen. Although mice reconstituted with nu/nu cells contained significantly higher numbers of Thy 1.2-positive cells in the BM than mice reconstituted with normal BM cells, smaller in vitro mitogen responses were generally given by the former mice. However, spleen and BM cells of both groups responded to phytohaemagglutinin and to concanavalin A. On the basis of comparisons with responses of BM and spleen cells from normal and ATx mice, anti-Thy 1.2 treatment of cells, and 1 g velocity sedimentation experiments, it is suggested that most mitogen responders in the BM of ATxXB mice are Thy 1.2-negative, pre-thymic T-cell precursors and, accordingly, that these cells are able to differentiate to some extent in the absence of the thymus.