Abstract A computational is presented for directly determining the minimal inhibitory concentration of antibiotics from a disc-diffusion susceptibility assay. The method is based on a numerical solution of a mathematical equation describing inhibition zones. An interactive simulation program was developed making the computations for determining MICs easy to carry out. In order to run the program one has to prepare a data file containing the values of the toxic radii and the appropriate parameters. Three of the parameters are fitted via the program itself. The method was tested with gramicidin S as the antibiotic and Bacillus subtilis as the microorganism. For this case the MIC was determined to be 1.55 μg/ml, which is similar to the MICs of gramicidin S for other bacili as determined by a liquid bioassay. Since the method is fast, efficient, and inexpensive, it could be used for the routine determination of MICs.