Rats were labeled with 3H-thymidine (3H-TdR) in utero and for 6 wk after birth in order to obtain 100% labeling of all bone marrow cells. Six weeks after the last 3H-TdR injection only cytokinetically resting cells were still labeled. At this time, the regenerative capacity of fractions obtained after centrifugation on a discontinuous albumin gradient was tested in 1200-R x-irradiated recipients, and the results were compared with the effect observed after transplantation of the same number of unfractionated bone marrow cells. One of the fractions obtained had a regeneratory capacity tenfold that of unfractionated cells. In contrast, when the response to a PHA stimulation test was evaluated, this fraction showed a decreased incorporation of 14C-TdR as compared to other fractions. In a second test system, fractions of bone marrow from 3H-TdR-labeled donors made hypoplastic by repeated injections of hydroxyurea were transfused into 1200-R x-irradiated recipients. Regenerative capacity was similar to that seen in the first experiment. The findings indicate a correlation between cytokinetically resting, small mononuclear cells and the regeneratory process in lethally x-irradiated recipients.