The metrological approach of IRMM in conducting laboratory comparisons is briefly explained. The three intercomparisons organised by IRMM so far presented: air filters spiked with 137Cs, metabolised 40K, 90Sr and 137Cs in milk powder, and naturally contained 226Ra, 234U and 238U in mineral waters. The determination of SI-traceable reference values, quite diffferent by method in these intercomparisons, is treated in examples. The evaluation and comparison of laboratory results are described in terms of relative deviation from the reference value and of the performance statistic "En number", where the second criterium underlines the necessary prudence of the participating laboratories for estimating the measurement uncertainty. No dependence of good or bad results from the method used in the determination of 90Sr in milk powder was observed.