In order to show equivalence in mass standards calibration among National Metrology Institutes of member countries of the 'Comité international des poids et mesures' (CIPM), key comparisons of mass standards have been carried out under the auspices of the 'Comité Consultatif pour la Masse et les Grandeurs Apparentées' (CCM). This key comparison on 50 kg standards in standard stainless steel was based on the decision of the CCM during the 11th meeting held in April 2008 at the 'Bureau International des Poids et Mesures' (BIPM). For this key comparison CENAM—Mexico acted as pilot laboratory, and NPL—United Kingdom accepted to be co-pilot laboratory. The aims of this key comparison were to compare the results obtained by NMIs in calibration of 50 kg stainless steel weights and to repeat the exercise realized in 2001–2002 with the key comparison identified as CCM.M-K3.