[en] To assess the safety of disposal of a radioactive waste-cement composite, the leaching of ¹³⁷Cs from a waste composite into a surrounding fluid was studied. Leaching tests were carried out in accordance with a method recommended by IAEA. The leachability was measured as a function of bentonite clay to cement ratio. The fraction of ¹³⁷Cs leached from a specimen of Portland cement is 0.03-0.13 at a leaching time of 400 d. Results presented in the paper are examples of data obtained in a 10 y mortar and concrete testing project, which will influence the design of the engineered trench system for a future Yugoslav radioactive waste storage center. (author) 7 refs.; 2 tabs