Mature blood cells have a finite life span and therefore continued production is required to maintain a constant level in tissues. Continuous replenishment is achieved by a constant feed-in from normal functioning hematopoietic stem cell compartments. The hematopoietic stem cell (HSC) system is characterized as follows: A totipotent hematopoietic stem cell (THSC) gives rise to all hematopoietic cells. Separate cells exist that are more differentiated progeny of THSC and are pluripotent for the myeloid system (PMSC: CFU-S, CFU-GEMM) and for the lymphoid system (PLSC). The PMSC gives rise to still more differentiated progenitor cells committed to erythrocytes (BFU-E, CFU-E), neutrophil-macrophages (CFU-NM) and megakaryocytes (CFU-MEG). One class of PMSC (CFU-S) is assayed in vivo. A second class of PMSC (CFU-GEMM), and most other types of progenitors (CFU-E, CFU-NM, CFU-MEG, etc.), are assayed in vitro. The mouse is the usual vehicle for the in vivo study of the CFU-S (colony-forming unit-spleen). Bone marrow cells are infused into lethally irradiated recipient mice, lodge in the spleen, and proliferate to form macroscopic colonies on the surface. There is no similar assay available in man, but the ability to clone mixed colonies (CFU-GEMM) in vitro allows one to study human pluripotent stem cells. In the presence of appropriate stimuli, CFU-GEMM form colonies in soft gel that contain granulocytes, erythroid cells, macrophages, and megakaryocytes. In addition to this class of PMSC, the differentiated progenitors that are committed to produce erythroid cells, neutrophils, megakaryocytes, or monocytes-macrophages also form colonies in vitro. A third method of determining the effect of antineoplastic agents on marrow cells is by use of the diffusion chamber (DC) culture technique. Marrow cells are inoculated into a diffusion chamber that is then implanted into the peritoneum of a mouse. After various time periods, chambers are removed and the number and differentiated cell types are determined. Modifications of the DC chamber technique include suspending marrow cells in a plasma clot or in agar within the chambers, which permits the growth of colonies within the chamber. A fourth method of assessing toxicity is by the use of the continuous long-term in vitro culture system. In this system, proliferation of marrow cells is supported by an adherent layer of marrow stromal cells.(ABSTRACT TRUNCATED AT 400 WORDS)