Summary: The method of labelled mitoses has been used to obtain estimates of the cell‐cycle time (and of its constituent phases) of erythroid precursors in normal adult rats, and in adult rats subjected to transient and protracted anaemias. It has been shown that considerable reductions occur in the cycle time in the anaemic rats, and that these reductions are due to decrease in the duration of both the S and G1 phases. The decreased cycle time is discussed in relation to the number of divisions that the erythroid precursors undergo during maturation.

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55Fe autoradiography of the peripheral red blood cells has been used to study the proliferation of the recognizable erythroid precursors in bled animals. The transit time of the recognizable erythroid precursors present in the bone marrow and labelled with 55Fe 6 hr before bleeding, remains unchanged, but the number of red cells produced by these precursors is significantly greater than normal. It is deduced that the increased red cell production is brought about by an increase in the number of divisions that the cells undergo during maturation and that a shortening in the red cell cycle time is implied. The possibility that the transit time of the progeny of cells differentiating into pro‐erythroblasts after bleeding may be shorter than the transit time of the precursors already differentiated before bleeding, is briefly discussed.

Calculations have been made of the change with time in the tritiated thymidine labelling index of erythroid cells in normal and anaemic rats, on the basis of two different models of the erythroid system. In the first model it is assumed that cells pass from one stage of maturation to the next at all phases of the cell cycle, whereas in the second model the cells can only progress to the next stage when they reach a certain point in the cell cycle. the changes in labelling index predicted on the basis of these two models are markedly different, especially in the non‐dividing stages of the system, and the change in labelling index as a function of time therefore provides an experimental method for distinguishing between the two models. the experimental data favours the model in which cells cross compartmental boundaries at all stages of maturation. Some important consequences of this model are discussed.

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