[en] The activity of a master solution of chemically purified ¹³⁷Cs was determined by the 4π(PC)-γ method with the use of ¹³⁴Cs as efficiency tracer. Details of the assay are described and some comments about the suitability of various counting conditions are given. Samples or sources of this ¹³⁷Cs were distributed among local laboratories for calibration purposes. Other samples were sent to the Bureau International des Poids et Mesures (BIPM), the International Atomic Energy Agency (IAEA) and Physikalisch-Technische Bundesanstalt (PTB) for comparison versus other ¹³⁷CS standards submitted to these agencies by international standardizing laboratories. The calibration factor of this laboratory's 4πγ ionization chamber is given, and the ratio for 661.6-keV emission rate ¹³⁷Cs activity is deduced. (author)