The Bureau International des Poids et Mesures organised an international comparison for the measurement of the radioactivity concentration of a 55Fe solution. The National Physical Laboratory was charged with the preparation and distribution of the solution and with the collation of the results. The active material was supplied by the National Accelerator Centre and purity-checked by the Laboratoire de Métrologie des Rayonnements Ionisants and the Physikalisch-Technische Bundesanstalt. The solution was distributed in December 1978 to eleven national and international laboratories. The numerous methods of measurement for activity and K X-ray emission rate are briefly described and the results obtained are presented in tabular and graphical form. In addition, the product of the K shell capture probability and the K shell fluorescence yield ( P K ω K) has been determined as 0.283±0.002.