The mononuclear phagocytes of the bone marrow can be classified into two cell types, promonocytes and monocytes. The present study was performed to establish whether the promonocytes are the progenitors of the monocytes and to determine the kinetic characteristics of the promonocytes and monocytes in the bone marrow compartment.

Both in vitro labeling studies with thymidine-3H and determination of the relative amount of DNA in the nuclei of individual cells showed that under normal steady-state conditions the promonocytes are proliferating cells and the monocytes, nondividing cells.

In vivo labeling studies provided further evidence that the promonocytes are the progenitor cells of the monocytes. During the first 24 hr after labeling, the promonocytes showed a constant high level of labeling (about 70%). The mean grain count of these cells decreased with time.

The labeling index of the monocytes of the bone marrow increased during the first 24 hr after in vivo labeling, but during the same period the mean grain counts remained almost constant, with values amounting to about half those of the promonocytes during the first 6 hr of the experiment.

The data concerning the labeling indices and the percentage distribution ratio of the promonocytes and monocytes in the bone marrow, and the labeling indices of the peripheral blood monocytes are used to construct a model population. The results lead to the conclusions that the promonocytes are multiplicative cells and that both daughter cells arising from the division of a promonocyte are monocytes.

The DNA-synthesis time found for the promonocytes is 13.6 hr. From this value, the average generation time was computed to be 19.5 hr.

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