Regression lines and their equations have been prepared for Neo‐Sensitabs® containing 26 different antimicrobials, using two different agar media and three different methods: usual without pre‐diffusion, 1 hour pre‐difusion and 1 + 20‐hour pre‐diffusion. The numerical value of the slope of a regression line indicates in mm the mean change in zone diameter for a 2‐fold change in MIC. The numerical value of the slope is increased when pre‐diffusion methods are employed, reaching its maximum with 1 + 20‐hour pre‐diffusion method. When comparing two agar media: Mueller Hinton and Danish Blood Agar, the difference between the slopes of the regression lines will be only slight if the same antimicrobial and the same method are used. There is a linear correlation between on a logarithmic scale the molecular weight of the antimicrobials contained in tablets or discs and, on a linear scale, the slope of the regression line pertaining to t