A review is presented of the experiments that resulted in the identification of a specific morphologic entity representing the pluripotential hemopoietic stem cell (HSC) in mouse bone marrow. This entity was subsequently discovered in concentrated HSC preparations from bone marrow of rats, monkeys, and humans. In the mouse, a set of physical parameters (of the HSC) has been collected which agree with its morphologic description. It was also shown that these physical properties, and a number of cell surface properties, do not enable a distinction between HSC and its immediate descendants, the G/M CFU 1 and the E-BFU. The factors that stimulate proliferation of these three cell types have been isolated from human leukocyte conditioned medium and mouse spleen conditioned medium and were partly purified and characterized. The information at present indicates that the three cell types respond to closely related, if not identical, factors. Direct counts of HSC in electron microscopic preparations of density gradient fractions of different enrichment have been compared with HSC values computed from spleen colony counts and f factors for rat and mouse marrow. A high degree of correlation was found between the two types of observations. The slopes of the regression lines for mouse marrow fractions, for concentrates of normal rat marrow, and for concentrates of cycling rat marrow were the same, namely, 0.5. The deviation of this value from the expected value of 1.0 is probably not due to the use of erroneous f values. It is proposed that the observed discrepancy may be due to heterogeneity of spleen colony forming cells, in that a proportion of them may not be pluripotential.