This study demonstrates the advantages and limitations of normalizing results for five serum proteins (IgG, IgA, IgM, transferrin and haptoglobin), analysed in liquid phase on ten different systems (open clinical chemistry and dedicated protein analysers). Seven sets of results from normal and pathological sera (without monoclonal proteins) were compared using: - calibrators supplied by each manufacturer; - - serial dilutions of a single stabilized pool of liquid serum. In addition to validating the quality of the stabilised serum, we have been able to identify: - significant variations in results using different analytical systems for the same sample; - a major reduction in these variations, often greater than 50%, through normalization using a common serum pool. In some two thirds of the cases, this reduction brought the degree of variation into an acceptable range.