The proliferating granulocyte precursor pool of rat bone marrow was labelled during DNA synthesis by continuous infusion and by single injection of /sup 3/H-thymidine (/sup 3/H-TdR), as well as by single injection of /sup 125/I-iododeoxyuridine (/sup 125/I-UdR). The appearance of neutrophilic granulocytes in the blood stream after these various labelling procedures was studied by autoradiography. Labelling patterns of blood neutrophils were identical during continuous infusion and after single injection of /sup 3/H-TdR, and 100 percent labelling of the blood compartment was achieved. This result indicated reutilization of DNA catabolites to occur in granulocytopoiesis leading to continuous availability of /sup 3/H-labelled DNA precursors even after a single injection of /sup 3/H-TdR. Attempts to suppress reutilization of label by infusion of cold thymidine 1 h after injection of /sup 3/H-TdR were unsuccessful. However, a change in the labelling pattern of blood neutrophils was seen after single injection of /sup 125/I-UdR, a DNA precursor poorly reutilized in comparison to /sup 3/H-TdR. This result provided further evidence for reutilization of DNA catabolites by the cell system investigated. A comprehensive discussion of the results indicates that thymidinemonophosphate is the biochemical level of reutilization in granulocytopoiesis.