MUC1 mucins are tumour markers that are frequently indicated and examined, particularly as part of the treatment of breast cancer. Relatively large differences were observed in external quality assessment (EQA) between the results that were obtained by different immunoassay technologies. Thus, we compared eight routinely employed immunoassay sets for the determination of MUC1 mucins in the serum: six closed automated systems (AxSYM, Centaur, ECi Vitros, Elecsys 2010, Immulite 2000 and Kryptor), and two IRMA kits (ELSA CIS and IRMA-mat Byk-Sangtec). Using all analytical systems, we measured identical groups of clinical samples complete with selected calibrator and control samples. The repeatability of measurements (presented as coefficients of variation) ranged from 0.7% (Kryptor) to 6.9% (Immulite 2000). Even though the cut-off values differ among various systems, no similar clinical efficacy appears to be attained. In the region of cut-off values, the highest specificity that was set as a standard was found for the AxSYM analyser, while the sensitivity was highest for the Elecsys 2010. Data from Bland-Altman differential plots suggest the presence of significant individual differences among individual samples, mainly in the region of high concentrations of MUC1 mucins. The parameters of Passing-Bablok regression show significant systematic differences between some of the analytical systems as well as an increase of the differences with increasing MUC1 mucin concentrations. The effect of the combination of antibodies used on the extent of differences among results obtained with individual systems is more obvious than the effect of the matrix of analysed materials.