SUMMARYCells proliferating in a local graft-versus-host reaction (GVHR) in the rat kidney were selectively labeled by local injection with radioactive precursors of deoxyribonucleic acid (DNA) while the blood vessels of the renal pedicle were temporarily occluded. The rats were killed sequentially and the amount of label in the local GVHR and other organs of the host was measured. When the label was given on day 5 the level within the local GVHR was sustained for at least 48 hr, whereas when given on day 7 it declined within 24 hr. There was no evidence that intact labeled cells migrated to lymphoid tissues of the host with the possible exception of the wall of the intestine, to which a minority of labeled cells migrated prior to their death. We thus conclude that most of the cells which proliferate in the local GVHR and their progeny died in situ about the 8th day under the conditions of these experiments.