AbstractIt is clear from extensive in vitro data that different subsets of lymphocytes stimulate and inhibit the growth of hematopoietic stem and progenitor cells. In order to clarify the complexity of the network between regulatory lymphocytes and hematopoietic target cells, we have examined the stimulatory and inhibitory effects derived from different lymphoid subsets. The regulatory influence of lymphocytes is transmitted mainly through the release of cytokines including the interleukins, granulocyte‐macrophage colony‐stimulating factor, tumor necrosis factor‐beta and the interferons, all of which have nonspecific effects on a variety of hematopoietic cells. Since these cytokines amplify the effects of other, more lineage‐specific cytokines (e.g., erythropoietin, thrombopoietin and granulocyte or macrophage colony‐stimulating factor) on the proliferation and differentiation of progenitor cells, the present review supports the conclusion that lymphoid subsets play a critical role in ensuring an optimal hematopoietic response to specifi