A procedure is described whereby 0.5 ml. suspensions containing 3-5 x 108 small lymphocytes per ml., nearly 50 per cent of which were labeled with H3 thymidine, have been prepared from rat thymus. Repeated doses of thymidine totaling 7-8 µc/Gm. and from two to three days were required to produce this percentage, and it is suggested that longer times would not be much more effective. Preliminary results indicate that cells of such suspensions injected in irradiated hosts are widely distributed, do not appear to disintegrate rapidly or to be massively ingested by the macrophages of the reticuloendothelial system, and are still present in good condition one and two days after injection.