Kinetin has generally been thought to be immobile in plants. This was confirmed in the case of laminar applications in this study, but not in regard to petiole, vein, or root applications. Radioactivity from kinetin-8-(14)C (Kn(*)) moved freely in the vascular system of several types of leaves. This movement was usually distal to the point of application and seemed to occur with the transpiration stream. Basipetal as well as acropetal translocation of radioactive kinetin was achieved in tobacco leaves. The translocated material was extracted from veinal tissue, shown to be radioactive, and to be able to retard senescence. Similar but less decisive results were obtained from agar blocks inserted into the vascular system of leaves receiving Kn(*) by petiole uptake.A bioassay employing disks from primary bean leaves was developed for the qualitative determination of substances like kinetin which possess the ability to retard chlorophyll breakdown and plant senescence. The use of radioactive kinetin provided a refinement in this bioassay because treated non-senescent areas could be correlated with exposed areas on radioautographs made from dried leaf disks.Root treatments showed that cotton seedlings did not take up Kn(*) but that similarly treated tobacco seedlings both absorbed and translocated the isotope readily.