Recently a commercial computer‐controlled image image analysis system (IAS) was introduced to measure automatically the diameters of inhibition zones in the agar diffusion test. However, there is little information on the precision of this method. In the present study clinical isolates ofSalmonellaspp. (N = 104),Escherichia coli(N = 100),Pasteurellaspp. (N = 99),Actinobacillus pleuropneumoniae(N = 85), porcine streptococci (N = 100), andStaphylococcus aureus(N = 95) were tested in the agar diffusion test, using nineteen different antibiotics in tablets. All inhibition zone diameters were first measured by a laboratory technician and then by the IAS. Although the zone diameters of all bacteria‐antibiotic combinations measured by the IAS and those measured by the laboratory technician showed a significant positive correlation, the size of the inhibition zone diameters measured by the technician and the IAS differed significantly in 59% of the combinations. However, these differences were very small and may have no clinical relevance. The IAS was also used to calculate minimum inhibitory concentrations (MIC values) from the zone diameters. In 82% of the bacteria‐antibiotic combinations MIC values calculated by the IAS showed a significant positive correlation with MIC values obtained with the reference agar dilution test. However, in 92% of the bacteria‐antibiotic combinations, the calculated MIC values differed significantly from the reference values. In some cases these differences were so large that they could be of clinical relevance. The IAS was unable to measure the diameter of inhibition zones of porcine streptococci properly, due to poor contrast. We concluded that when tablets are used as antibiotic carriers the IAS accurately measures the diameter of inhibition zones for bacteria species that give good contrast between the agar and bacterial growth. MIC values determined with the IAS were only indicative of those determined with the reference agar dilution test.