Regulation of granulopoiesis by activated splenic T lymphocyte subpopulations was investigated. The addition of syngeneic T cells activated by the mitogen concanavalin A or by mixed lymphocyte response (MLR) to normal bone marrow cultures stimulated granulopoiesis, while, in contrast, resting T cells did not. Functional analyses of Fluorescence Activated Cell Sorter (FACS) purified subpopulations of MLR activated T cells showed that both subsets enhanced granulocyte colony forming cells (CFUc) differentiation. Further studies using Ly 23+ T cells stimulated in vitro to suppress the primary generation of plaque forming cells (PFC) showed that these "suppressor cells" simultaneously enhanced CFUc differentiation. These results show that T cell "helper" function in hematopoietic regulation is not restricted to the inducer subset of T cells.