The hypothesis is proposed that regulation of hemopoiesis is largely accomplished by expansion or contraction of the committed stem cell compartments and that pluripotential stem cells are normally not involved or in cycle. The thesis appears supported by the fact that erythropoietin affects the committed red cell precursors, that 98% of marrow mitoses have been shown to occur in cells clearly recognizable as red or white cell precursors (while the pluripotential stem cells by definition are not so recognizable), and that it has been shown (by the spleen nodule assay) that the pluripotential stem cell compartment in the marrow cannot be readily expanded. The major objection to the proposed hypothesis are tritiated thymidine suicide data, which suggest that up to 20% of pluripotential stem cells may be constantly in cycle in some stains of mice. Preliminary experimental evidence supporting the hypothesis has been obtained: normal pluripotential stem cells which transfused into normal isologous mice are not lost as has been assumed but proliferate after irradiation, suggesting that it takes a special stimulus to "turn-on" the normally quiescent pluripotential stem cells.