It has been postulated by various investigators that cells which possess enzyme terminal deoxynucleotidyl transferase in the bone marrow of young adult experimental mammals and birds are T cell progenitors. Recently, an in vitro culture method which promotes the growth of rat bone marrow TdT-positive T cell progenitor cells has been established. This culture system requires a continuous presence of an adherent feeder layer from mouse bone marrow cells. TdT-positive cells from rat bone marrow selectively proliferate in the suspension phase of the culture media when a healthy mouse bone marrow feeder layer is present. We have also established a culture method for growing thymic epithelial cells in serum-free culture conditions. From these primary culture, a cell line (TEA3A1 cells) was established. TEA3A1 cells produce biological activity which stimulates the (3H)-TdR incorporation of ConA-stimulated thymocytes. This activity is different from I1-2. In this application we propose to examine the developmental potential of bone marrow TdT-positive cells and the functional capacity of various thymic stromal cells including TEA3A1 cells to support the growth and/or differentiation of TdT-positive cells.