The traceability routes for the calibration of brachytherapy sources supplied by Amersham International plc were recently summarized (Rossiter 1990) following continued interest in this topic by users. Such calibrations have depended on an ionometric comparison of the source output with one of a set of ⁶⁰Co reference sources. These reference sources have been calibrated for the measurement of brachytherapy sources by ionometric comparison, first with each other, then with an Amersham International ²²⁶Ra reference source which itself had been calibrated at NPL by comparison with the British National Radium Standard. The air kerma rate (formerly exposure rate) at 1 m distance from each ⁶⁰Co reference source was then calculated using the ionometric ratios, the widely accepted exposure rate constant for ²²⁶Ra and other minor corrections. The measurements have been extended to the important brachytherapy radionuclide ¹⁹²Ir. The results indicate a level of agreement between NPL and Amersham source output measurements which is within the NPL measurement uncertainties alone.