SUMMARYA large fraction of the total uptake of tracer cerium by washed resting yeast cells occurs quite rapidly (less than 15 min time). Optimum uptakes occur in acidic environments (pH 2.5-4.0). The cells show a relatively constant cerium binding capacity (about 3×10-4mg/mg dry wt), measured by radioactive tracer techniques after 2 hr contact time. The uptake is independent of temperature over the range from 4 to 38 C, is as high under anaerobic as under aerobic conditions, and is not affected by the common inhibitors of respiratory and glycolytic pathways. The cerium of labeled cells is rapidly removed by the hydrogen ion, complexing agents, and an excess of cerium cations. The data presented suggest cerium uptake by yeast involves primarily an adsorption of the element by some solid phase in or on the cell rather than an active transport process in which metabolic energy is required.