Thymocytes from normal mice strains as well as from genetically determined stem cell defective W/Wv anemic mice were cocultured with syngeneic (or congeneic) bone marrow cells. We assayed these cocultures for the proliferation of erythroid progenitor cell types (BFU-E and CFU- E) using the plasma clot technique. Results indicate that when concentrations of thymocytes were lower than bone marrow cells, significant suppression of erythroid growth was observed. However, when the concentration of thymocytes exceeded that of the bone marrow cells in culture (greater than 1:1), significant enhancement of erythroid growth was demonstrated. The W/Wv anemic bone marrow appears to respond to this interaction by enhancement at all concentrations of added normal thymocytes. The regulatory functions observed can be diminished by treatment of the thymocytes in vitro with anti-theta serum plus complement. Thus, we establish regulatory functions for anti-theta- sensitive regulatory cells (TSRC) with both positive (enhancement) and negative (suppression) components.