The diagnostic capacities of 4 commercial test systems were comparatively estimated for the detection of HBsAg, by applying a panel of samples with the established amino acid sequence of the "a" determinant of HBsAg. The Roche Elecsys HBsAg test system demonstrated the highest sensitivity - the maximum HBsAg concentrations were found in 19 of 31 cases. Escape mutations in the major hydrophilic region (MHR) of HBsAG (P120S, M133T) were responsible for differences in the sensitivity of 4 test systems by 10- to 40-fold. There were also samples that showed differences in the diagnostic capacities of the test systems to detect HBsAg, but without amino acid replacements in the area of the "a" determinant, which seems to be associated with amino acid replacements in other regions of HBsAg.