Tritiated thymidine was injected into normal rats at 6-hour intervals for 5.5 days, and the labelling of blood monocytes was recorded. Monocytes were identified on the basis of their cytochemical lysozyme activity, demonstrated by lysis of bacteria around the cells. No labelled monocytes were found one hour after the first injection, but a few labelled lysozyme-negative mononuclear blood cells were seen. 58% of the monocytes were labelled on the first day and 100% on the fourth day. Both unlabelled and heavily labelled monocytes left the circulation at random with a half-time of 12–13 h. The grain count data suggest that monocytes are the progeny of at least 2 succeeding precursor cells with generation times not exceeding 1.6 days and that part of the monocytes stay in a storage pool before being released into the blood.