Recent studies on the hematopoietic system have revealed that both constitutive and inducible hematopoiesis are regulated by complicated networks composed of various kinds of cytokines and that there are some general rules for hematopoietic cytokine biology. Further accumulation of knowledge about the regulation of proliferation and differentiation of hematopoietic cells by cytokines will serve to clarify the pathophysiology of hematological diseases, resulting in the development of therapeutic strategy for these diseases.