Lower land mosses uptake water and minerals from the atmosphere. They can collect metals polluting the air and radioactive fallout elements so they can be suitable for monitoring of these substances. Cesium ion uptake by Hypnum cupressiforme is studied by a radioactive tracer,134Cs. The quantity of cesium ion in different cellular locations and the capacity of ion uptake is determined. The total capacity is found to be several times 10−3mol g−1and is therefore of the same order of magnitude as the cation exchange capacity of ion exchangers. The kinetics and reversibility of the process is studied as well.