Microbial resistence to antibiotics is a very important parameter in the selection of a proper therapy and in the control of resistence spreading. One of the most used methods for measuring microbial susceptibility is the disk diffusion test. The test is based on diffusion of concentrated antibiotics from paper disk into agar. Concentration of antibiotics in the agar is dependent on distance from the center of the disc. Near antibiotic disc is concentration much higher due difficult permeation of antibiotics in agar. The diffused antibiotics then inhibit growth of sensitive strains. The zones are usually measured manually or by electronic calipers. Several aproaches for automatic inhibition zone detection have been introduced in recent years. Never- theless, most of the algorithms are based on a similar radial profile analysis. We have designed a novel image-processing algorithm for measuring the size of inhibition zones of antibiotics based on an analysis of corrected image and rated this image with multicriterial algorithms (based on an observation of the radius profile). The algorithm was tested on 100 clinical isolates, resulting in calculation accuracy of 89% (ratio of success computed radii). If we include alternative radii (to be selected manually), the precision of the calculation rises to 98% (tolerance deviations be- tween manual and automatic measurements were 2 mm). The achieved accuracy was independent of the culture medium (e.g., Muller-Hinton, blood agar, chocolate agar). The main advantage of the algorithm is the invariantness to the tested bacterial strain and culture medium. The new algorithm offers an alternative way to determine inhibition zones and evaluate antimicrobial susceptibility.