A large number of chemical agents are known which affect blood and blood-forming organs. The purpose of this volume is to review the sig- nificant advances made over the past several years regarding such chemical agents. The purification, biological action, and therapeutic implications of several widely used hematopoietic growth factors such as interleukin 3 (IL-3 or multi-CSF), granulocyte/macrophage colony stimulating factor (GM-CSF), granulocyte colony stimulating factor (G-CSF), colony stimu- lating factor (CSF-I or M-CSF), thrombopoietin, and erythropoietin are included in this volume. These factors are important in regulating several hematopoietic cell lines such as neutrophils, monocytes, eosinophils, macrophages, megakaryocytes, platelets, and erythrocytes. People are exposed daily to numerous toxic chemical substances present in our environment which produce a suppression of erythropoiesis, myelo- poiesis, lymphocytopoiesis, and megakaryocytopoiesis. Attempts have been made in this volume to assess the therapeutic role of some of the hematopoietic factors such as erythropoietin in the anemia of end stage renal disease, as well as colony stimulating factors in other hematopoietic abnormalities. In addition, some of the chemical factors in our environment which suppress major hematopoietic lineages stimulated by erythropoietin, macrophage colony stimulating factor, granulocyte colony stimulating factor, interleukin I-alpha, interleukin I-beta, and interleukins 2, 3, 4, 5, 6, 7, and 9 are also included. An updating of the mechanism of action of each of these factors on the major hematopoietic lineages is covered.