[en] A solution of ²⁰¹Tl was standardized and a sample was sent to the Bureau International des Poids et Mesures for comparative measurement. This indicated that the actual activity was about 7% lower than specified and the result was withdrawn from the registration tables. Trial measurements on ¹²⁵I were conducted in preparation for an international comparison of activity measurements of this radioisotope. ¹³⁹Ce, ¹⁰⁹Cd and ¹³⁷Cs sources were prepared for a number of users of radioisotopes. A new method for determining the activity of ⁵⁵Fe was investigated. A Monte Carlo program was written to calculate the interaction probability of photons in the detection systems used. 3 refs