Against 17 Gram-negative bacteria, excluding Proteus species, the difference between cefminox and cefotaxime was significant in terms of MIC distribution, in favour of cefotaxime, but was not significant in terms of ED50 distribution. Against 22 Proteus species, the difference between cefminox and cefotaxime was not significant in terms of MIC distribution, but was significant for ED50 distribution, in favour of cefminox. The difference between cefminox and cefoperazone was not significant for either MIC or ED50 distribution against 17 Gram-negative bacteria, but was significant against 22 Proteus species, in favour of cefminox. The distribution of rank of the ED50/MIC ratios for cefminox was significantly lower than those for cefotaxime and cefoperazone, indicating that cefminox showed lower ED50 values than expected from the MIC values.