SummaryThis report discusses an important source of discrepancy between results obtained with different systems of antibiotic sensitivity testing with the disc method : the lack of normalization of the number of sensitivity-categories and of the critical M.I.C. values which delimit these categories. The advantages and inconvenients of the three-and the four-category systems are briefly commented. The definition of uniformly adopted critical concentrations should be confided to a National Working Group, so that different testing systems can be adjusted accordingly.