1. The method of isotopic dilution was used to determine trace amounts of a heavy alkali metalcesium. 2. A new variation of the method of isotopic dilution using a radioactive isotope without a carrier was proposed. 3. It was established experimentally that the system magnesium dipicrylaminate — nitrobenzene can be used for the extraction-substoichiometric isolation of cesium from solutions of various concentrations. 4. A method was developed for determining ≥1μg/ml Cs in standard solutions by the method of repeated isotopic dilution with an average relative error of ≤2.5%.